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Posted by Vjero Fiala on December 6 2003 1:21am[Delete] [Edit]

Update one field with more fields from another table

Table A

+--------+-----------+
| A-num | text |
| 1 | |
| 2 | |
| 3 | |
| 4 | |
| 5 | |
+--------+-----------+

Table B:
+------+------+--------------+
| B-num| date | A-num |
| 22 | 01.08.2003 | 2 |
| 23 | 02.08.2003 | 2 |
| 24 | 03.08.2003 | 1 |
| 25 | 04.08.2003 | 4 |
| 26 | 05.03.2003 | 4 |

I will update field text in table A
with
UPDATE `Table A`,`Table B`
SET `Table A`.`text`=concat_ws('',`Table A`.`text`,`Table B`.`B-num`," from ",`Table B`.`date`,'/')
WHERE `Table A`.`A-num` = `Table B`.`A-num`

and come to this result
Table A
+--------+------------------------+
| A-num | text |
| 1 | 24 from 03 08 2003 / |
| 2 | 22 from 01 08 2003 / |
| 3 | |
| 4 | 25 from 04 08 2003 / |
| 5 | |
--------+-------------------------+
(only one field from Table B is accepted)

But i will come to this result
Table A
+--------+--------------------------------------------+
| A-num | text |
| 1 | 24 from 03 08 2003 |
| 2 | 22 from 01 08 2003 / 23 from 02 08 2003 / |
| 3 | |
| 4 | 25 from 04 08 2003 / 26 from 05 03 2003 / |
| 5 | |
+--------+--------------------------------------------+

Posted by Babu Ramesh on January 12 2004 11:33pm[Delete] [Edit]

Update column in a table whose values are not found in another table.

UPDATE TABLE_1 LEFT JOIN TABLE_2 ON TABLE_1.COLUMN_1= TABLE_2.COLUMN_2
SET TABLE_1.COLUMN = EXPR WHERE TABLE_2.COLUMN2 IS NULL

An outerjoin is performed based on the equijoin condition.
Records not matching the equijoin from table2 are marked with null.

This facilitates to update table1 column with expression whose corresponding value from table2 is returned as NULL

Posted by Adam Boyle on March 2 2004 2:28pm[Delete] [Edit]

It took me a few minutes to figure this out, but the syntax for UPDATING ONE TABLE ONLY using a relationship between two tables in MySQL 4.0 is actually quite simple:

update t1, t2 set t1.field = t2.value where t1.this = t2.that;

Posted by Neil Yalowitz on March 30 2004 7:56am[Delete] [Edit]

It should be noted that even simple applications of UPDATE can conflict with the 'safe mode' setting of the mysql daemon. Many server admins default the MySQL daemon to 'safe mode'.

If UPDATE gives an error like this:

"You are using safe update mode and you tried to update a table without...etc."

...then it may be that your .cnf file must be edited to disable safemode. This worked for me. In order for the change in the .cnf file to take effect, you must have permission to restart mysqld in the server OS environment. There is a page in the online documentation that explains safe mode entitled 'safe Server Startup Script'.

Posted by Csaba Gabor on May 26 2004 8:25am[Delete] [Edit]

Suppose you have a table where each row is associated with a certain group (For example, orders are associated with the customers placing them) where each item WITHIN the group has a distinct number (For example, each person my have a sequence of competition results - each person, therefore, has a 1st, 2nd, 3rd... competition).
If you would like to renumber items within their group so that each has the same baseline (say 0), here is an example way to proceed:

Create TEMPORARY Table Groups (Id INTEGER AUTO_INCREMENT PRIMARY KEY,
Name VARCHAR(31), GroupId VARCHAR(31), ValWithinGroup INTEGER);
INSERT INTO Groups VALUES (null, "Davy", "Boy", 2);
INSERT INTO Groups VALUES (null, "Mary", "Girl", 2);
INSERT INTO Groups VALUES (null, "Bill", "Boy", 5);
INSERT INTO Groups VALUES (null, "Jill", "Girl", -3);
INSERT INTO Groups VALUES (null, "Fred", "Boy", 3);

# Find the lowest value for each group
CREATE TEMPORARY TABLE GroupSum AS SELECT GroupId, MIN(ValWithinGroup)
AS baseVal FROM Groups GROUP BY GroupId;
# create an index so mySQL can efficiently match
ALTER TABLE GroupSum ADD UNIQUE (GroupId);
# finally, make the baseline adjustment
UPDATE Groups LEFT JOIN GroupSum USING (GroupId)
SET ValWithinGroup=ValWithinGroup-baseVal;
SELECT * FROM Groups;
# 1 Davy Boy 0
# 2 Mary Girl 5
# 3 Bill Boy 3
# 4 Jill Girl 0
# 5 Fred Boy 1
#Each group ("Boy", "Girl") now has a (lowest) ValWithinGroup entry of 0.

Notes: That index addition is necessary because on larger tables mySQL would rather die than figure to (internally) index a single column join.

I was not able, using mySQL 4.1.1, to do this as a subquery:
UPDATE Groups LEFT JOIN (SELECT GroupId, MIN(ValWithinGroup) AS baseVal FROM Groups GROUP BY GroupId) AS GrpSum USING (GroupId) SET ValWithinGroup=ValWithinGroup-baseVal;

Csaba Gabor

Posted by Michał Łukaszewski on June 10 2004 9:23pm[Delete] [Edit]

UPDATE Syntax with "on-line" updating value limitations.

I had a problem - a had to update a column "rate" but if the existince or new value is greater then 5 this "5" will be finally value in field.
So, I do it in one "magick" query ;)
Here an example:

"3" is a some value, from form or something

update item
set rate = case when round((rate+3)/2) < 6 then round((rate+3)/2) else 5 end
where id = 1 and rate <= 6;

greetings
pecado

Posted by Alex de Landgraaf on July 21 2004 10:07pm[Delete] [Edit]

You sometimes run into the problem that you want to replace a substring occuring in a column with a different string, without touching the rest of the string. The solution is surprisingly simple, thanks to MySQL:

UPDATE xoops_bb_posts_text
SET post_text=(
REPLACE (post_text,
'morphix.sourceforge.net',
'www.morphix.org'));

using the string function REPLACE, all items in the post_text column with 'morphix.sourceforge.net' get this substring replaced by 'www.morphix.org'. Ideal when writing a script is just too much effort.

Posted by Justin Swanhart on July 29 2004 5:32pm[Delete] [Edit]

Sometimes you have a lot of processes that could be updating a column value in a table. If you want to return the value before you updated it without using a seperate select (which unless you lock the table could return a different value than is updated) then you can use a mysql variable like this:

update some_table
set col = col + 1
where key = 'some_key_value'
and @value := col

The @value := col will always evaluate to true and will store the col value before the update in the @value variable.

You could then do

select @value;

in order to see what the value was before you updated it

Posted by Vladimir Petrov on December 9 2004 1:44pm[Delete] [Edit]

MySQL uses Watcom (Oracle) syntax for UPDATE, so it's possible to write something like:

update Table1 t1
join Table2 t2 on t1.ID=t2.t1ID
join Table3 t3 on t2.ID=t3.t2ID
set t1.Value=12345
where t3.ID=54321

Posted by Matt Ryan on February 16 2005 8:20pm[Delete] [Edit]

Here's a workaround for the update/subquery/cant do self table "bug"

Senario is, ID 8 has multiple records, only the last (highest) record needs to be changed

update t1 set c1 = 'NO'
where id='8'
order by recno desc limit 1

I would prefer update t1 set c1='NO' WHERE ID=8 AND RECNO = (SELECT MAX(RECNO) FROM T1 WHERE ID=8)

But that's not currently allowed

Posted by Bob Terrell on February 24 2005 4:34am[Delete] [Edit]

If you want to update a table based on an aggregate function applied to another table, you can use a correlated subquery, for example:

UPDATE table1 SET table1field = (SELECT MAX(table2.table2field) FROM table2 WHERE table1.table1field = table2.table2field)

This can be helpful if you need to create a temporary table storing an ID (for, say, a person) and a "last date" and already have another table storing all dates (for example, all dates of that person's orders).

Additional information on MySQL correlated subqueries is at http://dev.mysql.com/doc/mysql/en/correlated-subqueries.html

Posted by Ken Miller on April 6 2005 6:34am[Delete] [Edit]

I was looking at this example:

update item
set rate = case when round((rate+3)/2) < 6 then round((rate+3)/2) else 5 end
where id = 1 and rate <= 6;

I think it can be done simpler with LEAST:

update item
set rate = least(round((rate+3)/2), 5)
where id = 1 and rate <= 6;

(I could be wrong, but that looks like it ought to work.)

Posted by David Friedman on May 5 2005 8:58pm[Delete] [Edit]

The UPDATE can apparently be used to implement a semaphore (pardon my pseudocode):

while TRUE {
..UPDATE table SET value = 1
....WHERE value = 0 and name = 'name'
..if no. of rows affected > 0, break
..else wait and try again
}

The code above waits until the semaphore is "cleared" (value = 0) and then "sets" it (value = 1). When done, you "clear" the semaphore by

UPDATE table SET value = 0 WHERE name = 'name'

The assumption is that the UPDATE is "atomic" in that no concurrent access by another process can occur between testing and setting the value field.

Posted by Mohamed Hossam on May 9 2005 2:38am[Delete] [Edit]

[I have posted this in the Flow Control Functions page last year but I still see people asking how to update multiple rows. So, here it is again.]

A very server resources friendly method to update multiple rows in the same table is by using WHEN THEN (with a very important note).

UPDATE tbl_name SET fld2 = CASE fld1
WHEN val1 THEN data1
WHEN val2 THEN data2
ELSE fld2 END

The note is: do not forget ELSE. If you do not use it, all rows that are outside the range of your updated values will be set to blank!

Posted by Christian Hansel on July 1 2005 9:13am[Delete] [Edit]

If you wish to use an increment based on subset of a table you may combine UPDATE with Variables:

e.g. A table that contains entries of different categories, in which an internal order needs to represented ( lets say a table with busstops on different routes). If you add new entries or move stops from one route to another you will most likely want to increment the position of the busstop within this route. That's how you can do it

table busstops

id | route | busstop | pos
1 | 1 | A | 1
2 | 1 | B | 2
3 | 1 | C | 3
4 | 2 | C | 1
5 | 2 | D | 2
6 | 2 | A | 3
7 | 2 | E | 4
8 | 2 | F | 5
9 | 2 | G | 6
10 | 2 | H | 7

Moving D,E,F,G To route 1
SET @pos=(SELECT max(t1.pos) FROM busstops t1 WHERE t1.route = 1 );
UPDATE busstops SET pos = ( SELECT @pos := @pos +1 ), route =1 WHERE id IN (5,7,8,9)

I doubt this could be done otherwise since referencing the table you wish to update within the subquery creates circular references

After DELETE or UPDATE i.e. when a row of a subset is lost/deleted/moved away from it, the whole subset will need to be reordered. This can be done similarily :

SET @pos=0;
UPDATE busstops SET pos = ( SELECT @pos := @pos +1 ) WHERE route = 1 ORDER BY pos ASC

Chris H (chansel0049)

Posted by Anders Elton on November 24 2005 10:03pm[Delete] [Edit]

I experienced a weird issue converting from 4 to 5.

A is a normal table, B is a temporary table:
Worked in 4
update A, B set A.population=B.pop_count where A.id=B.id

In version 5, however, the above query only updated one element while still matching "all"

In 5 I had to do it like this:
update A RIGHT JOIN B on A.id=B.id set A.population=B.pop_count
Updates all population counts correctly.

[edit: RIGHT JOIN not LEFT JOIN...]

Posted by Jan Slauer on December 9 2005 1:37pm[Delete] [Edit]

I had the same problem after update from mysql 4.x.x to 5.x.x. I just exported all the tables to files via PhpMyAdmin
and imported them back. Now everything works fine.

Posted by [name withheld] on March 17 2006 1:09pm[Delete] [Edit]

Related to the post of Mohamed Hossam on May 9 2005 4:38am
A more general method to updtate more one row:

UPDATE table SET f1='foo', f2=
IF(f3=value,‘one’,IF(f3=value_bis,’two’,f2))
WHERE f5='afected'

This set the values of field 'f2' according to the values of field 'f3' in the rows field f5 'afected'.

Posted by Rafi B. on April 26 2006 6:44pm[Delete] [Edit]

Here is a way to use multiple tables in your UPDATE statement, but actually copying one row values into the other, meaning, we're using the same table:

UPDATE jobs AS toTable, jobs AS fromTable
SET
toTable.job_type_id = fromTable.job_type_id,
toTable.job_company_id = fromTable.job_company_id,
toTable.job_source = fromTable.job_source,
WHERE
(toTable.job_id = 6)
AND
(fromTable.job_id = 1)

--------------
Pretty cool. What I'm doing here is copying the information I need from the row where job_id=1 to the row where job_id=6, on the same table.

Posted by Christopher Marshall on June 7 2006 3:25pm[Delete] [Edit]

Adam Boyle's commment above was just what I was trying to do, update one table based on a relationship between that table and another. His example was:

update t1,t2 set t1.field=t2.value where t1.this=t2.that;

That strikes me as an elegant syntax. Here is the closest I could come up with for doing that on Oracle:

update t1 set t1.field=(select value from t2 where t1.this=t2.that) where t1.this in (select that from t2);

That strikes me as convoluted by comparison.

Posted by venky kris on June 8 2006 2:06pm[Delete] [Edit]

Just following up on Matt Ryan's Post

Matt Ryan Writes :
>>Here's a workaround for the update/subquery/cant do self >>table "bug"

>>Senario is, ID 8 has multiple records, only the last
>>(highest) record needs to be changed

>>update t1 set c1 = 'NO'
>>where id='8'
>>order by recno desc limit 1

You can also accomplish the same by the following query :

update t1 , (select id ,max(recno) as recno from t1 where id=8 group by recno) tt
set t1.c1 = 'NO'
where tt.id=t1.id and
t1.recno=tt.recno

Comments are welcome.

Posted by Paul Decowski on August 1 2006 9:43am[Delete] [Edit]

Regarding Justin Swanhart's comment about retrieving a field's value in UPDATE query.

> update some_table
> set col = col + 1
> where key = 'some_key_value'
> and @value := col

> The @value := col will always evaluate to true and will store the col value before the update in the @value variable.

In fact, in won't if `col` is NULL (0, empty string etc.) - then the condition is not met and the update query won't be processed. The correct condition would be:

AND ((@value := `col`) OR (1 = 1))

It was very helpful to me anyway. Thx Justin!

Posted by Barry Shantz on October 24 2006 9:40pm[Delete] [Edit]

To update a column of a table with a rank based on subsets of data, the IF() function does a wonderful job.

A summary table (in this case created to hold summary counts of other genealogy data, based on the two fields that make up the PRIMARY key) often contains unique key fields and one or more summary totals (Cnt in this case). Additional ranking fields in the summary table can be easily updated to contain rankings of the Cnt field using the IF function and
local variables.

Table DDL:

CREATE TABLE `countsbyboth` (
`SurnameID` int(11) unsigned NOT NULL default '0',
`GedID` int(11) unsigned NOT NULL default '0',
`Cnt` int(11) unsigned NOT NULL default '0',
`sRank` int(11) unsigned NOT NULL default '0',
`nRank` int(11) unsigned NOT NULL default '0',
PRIMARY KEY (`SurnameID`,`GedID`),
KEY `SurnameID` (`SurnameID`,`Cnt`),
KEY `GedID` (`GedID`,`Cnt`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

After populating the table with rows containing key and summary data (and leaving the rank field(s) to be updated in
a subsequent step), the rank fields can be updated using syntax similar to the following:

update countsbyboth set srank=0, nrank=0;
set @rnk:=1, @gedid=0;
update countsbyboth
set srank=if(@gedid=(@gedid:=gedid), (@rnk:=@rnk+1),(@rnk:=1))
order by gedid desc, cnt desc;
set @rnk:=1, @snmid=0;
update countsbyboth
set nrank=if(@snmid=(@snmid:=surnameid), (@rnk:=@rnk+1),(@rnk:=1))
order by surnameid desc, cnt desc;

Query OK, 11752 rows affected (0.08 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 11752 rows affected (0.24 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 11752 rows affected (0.19 sec)

It looks convoluted, but is really quite simple. The @rnk variable needs to be initialized, and the keyval variable (in this case @gedid or @snmid) needs to be set to a value that will not be matched by the first record. The IF() function checks the previous key value (left side) against the current key value (right side), and either increments the @rnk variable when the desired key value is the same as the previous records, or reset the @rnk variable to 1 when the key value changes.

This can be easily extended to accomodate ranking on more than one key value, and does not require sub-selects that take considerable resources for a large table.

This example intentionally assigns different ranks to equal values of Cnt for a given key, to facilitate reporting where column headings contain the rank value.

Posted by Barry Shantz on October 24 2006 9:51pm[Delete] [Edit]

In the previous example, if the @rnk value is initialed with
set @rnk:=0
rather than
set @rnk:=1 ,
then it won't matter whether or not the first record's key value matches the 'key value' @gedid variable.

Posted by Jon Meredith on October 31 2006 5:09pm[Delete] [Edit]

Thanks for Justin Swanhart/Paul Decowski tip. As of 5.0.18 it looks like the optimiser has been improved so the

AND ((@value := `col`) OR (1 = 1))

gets optimised out as 'true' and @value is left as NULL after the update.

I got it to work again by rewriting as

update some_table
set col = col + 1
where key = 'some_key_value'
and ((@value := col) IS NULL OR (@value := col) IS NOT NULL)

So you get a true value either way and value will get set. Be careful what you put on the right-hand-side as it could get evaluated twice.

Posted by Dewey Gaedcke on December 27 2006 8:34pm[Delete] [Edit]

Above in the docs, it says "you cannot update a table and select from the same table in a subquery"

This is true but there are two simple ways around this limit.
1) nest the subquery 2 deep so it is fully materialized before the update runs. For example:
Update t1 set v1 = t3.v1 where id in
(select t2.id, t2.v1 from (select id, v1 from t1) t2) t3

2) use a self join rather than a subquery

Posted by Lars Aronsson on March 9 2007 10:07pm[Delete] [Edit]

Oracle databases has a keyword NOWAIT that can be used with UPDATE, causing the update to abort if it would get stuck waiting for locks. This keyword is not available in MySQL. Just letting you know, so you can stop looking for it.

Posted by John Batzel on March 16 2007 1:58pm[Delete] [Edit]

The UPDATE 'bug' mentioned above is apparently related to upgrading from 4.x to 5.0x. The indexes are slightly different formats, and it breaks *some* things. myisamchk/check table won't fix this. Dropping and re-adding the indexes will. (And dumping the table to file and reloading it is just recreating the indexes with lots more IO than you need to do.)

Posted by James Goatcher on March 16 2007 9:28pm[Delete] [Edit]

This example/tip/bug-report uses MySQL version 5.0.19.

When updating one table using values obtained from another table, the manual describes the "update table1, table2" syntax, but does not delve into the correlated subquery approach very much. It also does not point out a VERY important execution difference.

Consider the following script:
======================================================
drop table if exists test_1;
drop table if exists test_2;

CREATE TABLE test_1 (
col_pk integer NOT NULL,
col_test integer
);

alter table test_1 add PRIMARY KEY (col_pk);

CREATE TABLE test_2 (
col_pk_join integer NOT NULL,
col_test_new integer
);

insert into test_1 (col_pk, col_test) values ( 1, null );
insert into test_1 (col_pk, col_test) values ( 2, null );
commit;

insert into test_2 (col_pk_join, col_test_new) values ( 1, 23 );
insert into test_2 (col_pk_join, col_test_new) values ( 1, 34 );
insert into test_2 (col_pk_join, col_test_new) values ( 2, 45 );
commit;

select * from test_1;
select * from test_2;

# This update should NOT work, but it does.
UPDATE test_1 t,
test_2 tmp
set t.col_test = tmp.col_test_new
where t.col_pk = tmp.col_pk_join;
commit;

select * from test_1;
======================================================

The output of the select and update statements is:

+--------+----------+
| col_pk | col_test |
+--------+----------+
| 1 | NULL |
| 2 | NULL |
+--------+----------+
2 rows in set

+-------------+--------------+
| col_pk_join | col_test_new |
+-------------+--------------+
| 1 | 23 |
| 1 | 34 |
| 2 | 45 |
+-------------+--------------+
3 rows in set

Query OK, 2 rows affected
Rows matched: 2 Changed: 2 Warnings: 0

Query OK, 0 rows affected

+--------+----------+
| col_pk | col_test |
+--------+----------+
| 1 | 23 |
| 2 | 45 |
+--------+----------+
2 rows in set

Note that the update did NOT produce any errors or warnings. It should have. Why? Because a join on value 1 produces two values from table test_2. Two values cannot fit into a space for one. What MySQL does in this case is use the first value and ignore the second value. This is really bad in my opinion because it is, in essence, putting incorrect data into table test_1.

Replace the update statement above with:
UPDATE test_1 t1
set t1.col_test = (
select col_test_new
from test_2 t2
where t1.col_pk = t2.col_pk_join
)
;

This will produce the appropriate error for the given data:
"ERROR 1242 : Subquery returns more than 1 row"
and will not perform any update at all, which is good (it protects table test_1 from getting bad data).

Now if you have different data........if you comment out one of the "1" values inserted into table test_2 and use the correlated subquery update instead of the multi-table update, table test_1 will get updated with exactly what you expect.

The moral of this example/tip/bug-report: do not use the multi-table update. Use the correlated subquery update instead. It's safe. If you keep getting an error when you think you shouldn't, you either have bad data in your source table or you need to rework your subquery such that it produces a guaranteed one-row result for each destination row being updated.

The reason I call the multi-table update a bug is simply because I feel it should produce the same or similar error as the correlated subquery update. My hope is that MySQL AB will agree with me.

Posted by Luciano Fantuzzi on March 23 2007 2:32am[Delete] [Edit]

Este sencillo script permite recrear el indice de una columna de forma automatica.
Nota: Si una columna tiene una restriccion NOT NULL, sera necesario usar primero 'ALTER TABLE' para quitarle temporalmente la restriccion.

/* INICIO del script */

#En caso de tener con NOT NULL alguna columna (Ejemplo)
ALTER TABLE MiTabla CHANGE columna
columna MEDIUMINT UNSIGNED DEFAULT NULL;

#Cambio todos los valores a NULL (para que no haya riesgo de valores duplicados con restricciones UNIQUE)
UPDATE MiTabla SET columna=NULL;

#Declaro una variable como contador (puede ser 1,2,3... o el num desde donde queremos empezar)
SET @c:=1;

#Consulta
UPDATE MiTabla SET columna=(SELECT @c:=@c+1);

#Ahora podemos usar ALTER TABLE nuevamente si queremos cambiar la columna a NOT NULL (en caso de que la hayamos cambiado)

/* FIN del script */

Tengan en cuenta de que los indices principales (los declarados como PRIMARY KEY, por ejemplo, o los que se usan para linquear tablas) NO DEBERIAN CAMBIARSE, ya que se estropearian los vinculos entre las tablas! Esto podria evitarse declarando las claves foraneas (FOREIGN KEY) de las tablas de linqueo con el valor ON UPDATE CASCADE (lo que al actualizar los indices refrescaria los links entre las tablas).

Posted by Marc Vos on July 18 2007 1:15pm[Delete] [Edit]

Here's the easy way to update a column in a table using values from other tables.

update db1.a, (
select distinct b.col1, b.col2
from db2.b, db2.c, db2.d
where b.col1<>'' and d.idnr=b.idnr and c.user=d.user and c.role='S'
order by b.col1) as e
set a.col1 = e.col1
where a.idnr = e.col1

The point is that every select statement returns a table. Name the result and you can access its columns. In this example I called the result 'e'.

Posted by Richard Bronosky on September 5 2007 5:05pm[Delete] [Edit]

Marc Vos led me to a solution to a problem that has been troubling me for a long time. As a DBA I often have to support application developers who need to have data I control presented in a specific manner. This always results in a table based on their needs and populating the columns with data from existing tables. Usually it something like 15 columns from table A, 5 from table B, 30 from table c, and 230 from table d. In the past I have done this with either a series of "create temporary table t1 as select ... join ..." statements until I get the right set of columns.

I never could figure out how to set the value of multiple columns with nesting a select statement dedicated to each column. Now I've got it. I'm attaching a transcript of doing it both ways. The statements use the tables that already exist in the mysql schema (at least in 5.0), so you can easily recreate this on your box in a test schema.

--------------
DROP TABLE IF EXISTS test
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
CREATE TABLE test (t_id INT,k_id INT, t_name CHAR(64), t_desc TEXT) AS
SELECT help_topic_id AS t_id, help_keyword_id AS k_id, NULL AS t_name, NULL AS t_desc FROM mysql.help_relation LIMIT 10
--------------

Query OK, 10 rows affected (0.01 sec)
Records: 10 Duplicates: 0 Warnings: 0

--------------
SELECT * FROM test
--------------

+------+------+--------+--------+
| t_id | k_id | t_name | t_desc |
+------+------+--------+--------+
| 0 | 0 | NULL | NULL |
| 327 | 0 | NULL | NULL |
| 208 | 1 | NULL | NULL |
| 409 | 2 | NULL | NULL |
| 36 | 3 | NULL | NULL |
| 388 | 3 | NULL | NULL |
| 189 | 4 | NULL | NULL |
| 169 | 5 | NULL | NULL |
| 393 | 6 | NULL | NULL |
| 17 | 7 | NULL | NULL |
+------+------+--------+--------+
10 rows in set (0.00 sec)

--------------
######
## This is the elegant single select solution! ##
######
UPDATE test AS t, (SELECT * FROM mysql.help_topic) AS h SET
t.t_name=h.name,
t.t_desc=substr(h.url,1-locate('/',reverse(h.url)))
WHERE t.t_id=h.help_topic_id
--------------

Query OK, 10 rows affected (0.04 sec)
Rows matched: 10 Changed: 10 Warnings: 0

--------------
SELECT * FROM test
--------------

+------+------+------------------+---------------------------+
| t_id | k_id | t_name | t_desc |
+------+------+------------------+---------------------------+
| 0 | 0 | JOIN | join.html |
| 327 | 0 | SELECT | select.html |
| 208 | 1 | REPEAT LOOP | repeat-statement.html |
| 409 | 2 | ISOLATION | set-transaction.html |
| 36 | 3 | REPLACE INTO | replace.html |
| 388 | 3 | LOAD DATA | load-data.html |
| 189 | 4 | CREATE FUNCTION | create-function.html |
| 169 | 5 | CHANGE MASTER TO | change-master-to.html |
| 393 | 6 | CHAR | string-type-overview.html |
| 17 | 7 | SHOW COLUMNS | show-columns.html |
+------+------+------------------+---------------------------+
10 rows in set (0.03 sec)

--------------
DROP TABLE IF EXISTS test
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
CREATE TABLE test (t_id INT,k_id INT, t_name CHAR(64), t_desc TEXT) AS
SELECT help_topic_id AS t_id, help_keyword_id AS k_id, NULL AS t_name, NULL AS t_desc FROM mysql.help_relation LIMIT 10
--------------

Query OK, 10 rows affected (0.01 sec)
Records: 10 Duplicates: 0 Warnings: 0

--------------
SELECT * FROM test
--------------

+------+------+--------+--------+
| t_id | k_id | t_name | t_desc |
+------+------+--------+--------+
| 0 | 0 | NULL | NULL |
| 327 | 0 | NULL | NULL |
| 208 | 1 | NULL | NULL |
| 409 | 2 | NULL | NULL |
| 36 | 3 | NULL | NULL |
| 388 | 3 | NULL | NULL |
| 189 | 4 | NULL | NULL |
| 169 | 5 | NULL | NULL |
| 393 | 6 | NULL | NULL |
| 17 | 7 | NULL | NULL |
+------+------+--------+--------+
10 rows in set (0.00 sec)

--------------
######
## This is the nasty one select for each column that needs to be updated method! ##
######
UPDATE test AS t SET
t.t_name=(SELECT name FROM mysql.help_topic WHERE t.t_id=help_topic_id),
t.t_desc=(SELECT substr(url,1-locate('/',reverse(url))) FROM mysql.help_topic WHERE t.t_id=help_topic_id)
--------------

Query OK, 10 rows affected (0.00 sec)
Rows matched: 10 Changed: 10 Warnings: 0

--------------
SELECT * FROM test
--------------

+------+------+------------------+---------------------------+
| t_id | k_id | t_name | t_desc |
+------+------+------------------+---------------------------+
| 0 | 0 | JOIN | join.html |
| 327 | 0 | SELECT | select.html |
| 208 | 1 | REPEAT LOOP | repeat-statement.html |
| 409 | 2 | ISOLATION | set-transaction.html |
| 36 | 3 | REPLACE INTO | replace.html |
| 388 | 3 | LOAD DATA | load-data.html |
| 189 | 4 | CREATE FUNCTION | create-function.html |
| 169 | 5 | CHANGE MASTER TO | change-master-to.html |
| 393 | 6 | CHAR | string-type-overview.html |
| 17 | 7 | SHOW COLUMNS | show-columns.html |
+------+------+------------------+---------------------------+
10 rows in set (0.00 sec)

Bye

Posted by Joris Kinable on April 30 2008 9:34am[Delete] [Edit]

Updating multiple fields based on query results can be quite expensive if the same query has to be executed multiple times. Imagine the following table:

summary(X,A,B,C,D) and a query which returns: (X,E,F) and you want to update the summary table fields C and D with the values of E and F:

Summary: (1,2,3,0,0),(10,12,13,0,0) and query result: (1,4,5),(10,14,15) should result in the updated summary table: (1,2,3,4,5,6),(10,11,12,13,14,15)

BAD SOLUTION (same query is evaluated twice!):

UPDATE summary SET C=(SELECT E FROM (query) q WHERE summary.X=q.X), D=(SELECT F FROM (query) q WHERE summary.X=q.X)

GOOD SOLUTION (query is only evaluated once):

UPDATE summary AS t, (query) AS q SET t.C=q.E, t.D=q.F WHERE t.X=q.X

Posted by Nigel Smith on September 16 2008 9:31am[Delete] [Edit]

Example of updating a table using a group selection from another table:-
update tableA,
(
select idTableA,min(valueField) as minV from tableB group by idTableA
) as T
set tableA.minValue=minV where tableA.idTableA=T.idTableA

Posted by Roger Morris on September 23 2008 4:04am[Delete] [Edit]

To swap two values in a single table. If you need to keep the lower value in a certain column:
mysql> select * from test;

+-------+------+-------+-------+
| index | name | item1 | item2 |
+-------+------+-------+-------+
| 1 | one | 25 | 50 |
| 2 | two | 75 | 40 |
| 3 | one | 35 | 60 |
| 4 | four | 100 | 80 |
+-------+------+-------+-------+
4 rows in set (0.00 sec)

### (@olditem1:=item1) will assign the value of item1 *before* the update.

mysql> update test set item1=item2,item2=@olditem1 where (@olditem1:=item1) and item1>item2;
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0

mysql> select * from test;
+-------+------+-------+-------+
| index | name | item1 | item2 |
+-------+------+-------+-------+
| 1 | one | 25 | 50 |
| 2 | two | 40 | 75 |
| 3 | one | 35 | 60 |
| 4 | four | 80 | 100 |
+-------+------+-------+-------+
4 rows in set (0.00 sec)

Posted by Enrico Modanese on September 2 2009 7:48am[Delete] [Edit]

Following the post of James Goatcher (object: ORDER BY in multi-table UPDATE) I'd like to resume the argument and related work-around:

1. multi-table UPDATE doesn't support ORDER BY (as written in documentation)

2. multi-table UPDATE retrieving more than 1 row for every row to be updated, will perform only 1 update with the first found value and wont send any message about following skipped values (I don't know if it should be called an error)

3. first work-around (+quick -secure): be sure that the joined tables are ordered to offer as first the correct value

4. second work-around (-quick +secure): use a subselect for the value to be set [ x=(SELECT yy FROM ... ORDER BY... LIMIT 1) ] as shown in the preceding example of James Goatcher (please note the use of LIMIT)

Hope this will help, Enrico

Posted by Pavel Tishkin on January 26 2010 7:39pm[Delete] [Edit]

UPDATE some_table as bm
SET bm.i_ordi=(SELECT @a:=@a+1)
WHERE bm.i_type=1 AND (@a:=IFNULL(@a,-2)+1)<>'1'
ORDER BY bm.i_create_ts;

Sorting AND set auto_increment order

Posted by Anto Justus on June 2 2010 9:46am[Delete] [Edit]

you can try this type of query :

UPDATE dt_log AS t,
(
SELECT max(el_count)+1 as maxcount
FROM dt_log where dt_nameid IN ('1','2','3','4')
) AS h
SET t.dt_rej = h.maxcount
WHERE t.dt_edate = '0000-00-00 00:00:00'
AND t.dt_nameid IN ('1','2','3','4')

get the max and update..

Posted by Dave Scotese on October 11 2010 11:49pm[Delete] [Edit]

When I executed a correlated subquery:

UPDATE T1 SET f1= (SELECT COUNT(*) FROM T2 WHERE T2.f2=T1.f2)

to update one table with aggregates from another, it took 3 seconds to do 50 records and about 57 seconds to do 800 records.

When I used a non-correlated subquery:

UPDATE T1 INNER JOIN (SELECT COUNT(*) AS c FROM T2 GROUP BY f2) t USING(f2) SET f1=t.c

it did 76 records in 177 ms, and 350 records in 177 ms and 2800 records in 250 ms.

I believe that a correlated subquery is executed once for each result of the outer query, whereas a JOIN to a non-correlated subquery executes the inner query only once.

Posted by Misha B on April 21 2011 4:49pm[Delete] [Edit]

Change values between two and more columns. In result, ufter update, columns will have values from after columns
column1 = column2, column2 = column1

UPDATE
table1
SET
column1 = (@v := column1), column1 = column2, column2 = @v;

Posted by Ajmer Phull on February 2 2012 7:24am[Delete] [Edit]

Hopefully this will be useful to someone else, like it was for me when I had to perform data cleansing and enhancing badly designed databases. This can also be helpful for replacing data in fields with ID's when normalising databases.

The following will update a field (field9 which is empty) in TABLE1 with data from a field (field9) in TABLE3 using joins with TABLE2 and TABLE3. I have made up the WHERE & AND conditions to show this example.

UPDATE table1 t1
JOIN table2 t2 ON t1.field1 = t2.field1
JOIN table3 t3 ON (t3.field1=t2.field2 AND t3.field3 IS NOT NULL)
SET t1.field9=t3.field9
WHERE t1.field5=1
AND t1.field9 IS NULL