13.2.9 SELECT Statement

13.2.9.1 SELECT ... INTO Statement

13.2.9.2 JOIN Clause

13.2.9.3 UNION Clause

SELECT
    [ALL | DISTINCT | DISTINCTROW ]
    [HIGH_PRIORITY]
    [STRAIGHT_JOIN]
    [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
    [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr [, select_expr] ...
    [into_option]
    [FROM table_references
      [PARTITION partition_list]]
    [WHERE where_condition]
    [GROUP BY {col_name | expr | position}
      [ASC | DESC], ... [WITH ROLLUP]]
    [HAVING where_condition]
    [ORDER BY {col_name | expr | position}
      [ASC | DESC], ...]
    [LIMIT {[offset,] row_count | row_count OFFSET offset}]
    [PROCEDURE procedure_name(argument_list)]
    [into_option]
    [FOR UPDATE | LOCK IN SHARE MODE]

into_option: {
    INTO OUTFILE 'file_name'
        [CHARACTER SET charset_name]
        export_options
  | INTO DUMPFILE 'file_name'
  | INTO var_name [, var_name] ...
}

export_options:
    [{FIELDS | COLUMNS}
        [TERMINATED BY 'string']
        [[OPTIONALLY] ENCLOSED BY 'char']
        [ESCAPED BY 'char']
    ]
    [LINES
        [STARTING BY 'string']
        [TERMINATED BY 'string']
    ]

SELECT is used to retrieve rows selected from one or more tables, and can include UNION statements and subqueries. See Section 13.2.9.3, “UNION Clause”, and Section 13.2.10, “Subqueries”.

The most commonly used clauses of SELECT statements are these:

• Each select_expr indicates a column that you want to retrieve. There must be at least one select_expr.

• table_references indicates the table or tables from which to retrieve rows. Its syntax is described in Section 13.2.9.2, “JOIN Clause”.

• SELECT supports explicit partition selection using the PARTITION clause with a list of partitions or subpartitions (or both) following the name of the table in a table_reference (see Section 13.2.9.2, “JOIN Clause”). In this case, rows are selected only from the partitions listed, and any other partitions of the table are ignored. For more information and examples, see Section 22.5, “Partition Selection”.

  SELECT ... PARTITION from tables using storage engines such as MyISAM that perform table-level locks (and thus partition locks) lock only the partitions or subpartitions named by the PARTITION option.

  For more information, see Section 22.6.4, “Partitioning and Locking”.

• The WHERE clause, if given, indicates the condition or conditions that rows must satisfy to be selected. where_condition is an expression that evaluates to true for each row to be selected. The statement selects all rows if there is no WHERE clause.

  In the WHERE expression, you can use any of the functions and operators that MySQL supports, except for aggregate (group) functions. See Section 9.5, “Expressions”, and Chapter 12, Functions and Operators.

SELECT can also be used to retrieve rows computed without reference to any table.

For example:

mysql> SELECT 1 + 1;
        -> 2

You are permitted to specify DUAL as a dummy table name in situations where no tables are referenced:

mysql> SELECT 1 + 1 FROM DUAL;
        -> 2

DUAL is purely for the convenience of people who require that all SELECT statements should have FROM and possibly other clauses. MySQL may ignore the clauses. MySQL does not require FROM DUAL if no tables are referenced.

In general, clauses used must be given in exactly the order shown in the syntax description. For example, a HAVING clause must come after any GROUP BY clause and before any ORDER BY clause. The INTO clause, if present, can appear in any position indicated by the syntax description, but within a given statement can appear only once, not in multiple positions. For more information about INTO, see Section 13.2.9.1, “SELECT ... INTO Statement”.

The list of select_expr terms comprises the select list that indicates which columns to retrieve. Terms specify a column or expression or can use *-shorthand:

• A select list consisting only of a single unqualified * can be used as shorthand to select all columns from all tables:

  SELECT * FROM t1 INNER JOIN t2 ...

• tbl_name.* can be used as a qualified shorthand to select all columns from the named table:

  SELECT t1.*, t2.* FROM t1 INNER JOIN t2 ...

• Use of an unqualified * with other items in the select list may produce a parse error. For example:

  SELECT id, * FROM t1

  To avoid this problem, use a qualified tbl_name.* reference:

  SELECT id, t1.* FROM t1

  Use qualified tbl_name.* references for each table in the select list:

  SELECT AVG(score), t1.* FROM t1 ...

The following list provides additional information about other SELECT clauses:

• A select_expr can be given an alias using AS alias_name. The alias is used as the expression's column name and can be used in GROUP BY, ORDER BY, or HAVING clauses. For example:

  SELECT CONCAT(last_name,', ',first_name) AS full_name
    FROM mytable ORDER BY full_name;

  The AS keyword is optional when aliasing a select_expr with an identifier. The preceding example could have been written like this:

  SELECT CONCAT(last_name,', ',first_name) full_name
    FROM mytable ORDER BY full_name;

  However, because the AS is optional, a subtle problem can occur if you forget the comma between two select_expr expressions: MySQL interprets the second as an alias name. For example, in the following statement, columnb is treated as an alias name:

  SELECT columna columnb FROM mytable;

  For this reason, it is good practice to be in the habit of using AS explicitly when specifying column aliases.

  It is not permissible to refer to a column alias in a WHERE clause, because the column value might not yet be determined when the WHERE clause is executed. See Section B.3.4.4, “Problems with Column Aliases”.

• The FROM table_references clause indicates the table or tables from which to retrieve rows. If you name more than one table, you are performing a join. For information on join syntax, see Section 13.2.9.2, “JOIN Clause”. For each table specified, you can optionally specify an alias.

  tbl_name [[AS] alias] [index_hint]

  The use of index hints provides the optimizer with information about how to choose indexes during query processing. For a description of the syntax for specifying these hints, see Section 8.9.4, “Index Hints”.

  You can use SET max_seeks_for_key=value as an alternative way to force MySQL to prefer key scans instead of table scans. See Section 5.1.7, “Server System Variables”.

• You can refer to a table within the default database as tbl_name, or as db_name.tbl_name to specify a database explicitly. You can refer to a column as col_name, tbl_name.col_name, or db_name.tbl_name.col_name. You need not specify a tbl_name or db_name.tbl_name prefix for a column reference unless the reference would be ambiguous. See Section 9.2.2, “Identifier Qualifiers”, for examples of ambiguity that require the more explicit column reference forms.

• A table reference can be aliased using tbl_name AS alias_name or tbl_name alias_name. These statements are equivalent:

  SELECT t1.name, t2.salary FROM employee AS t1, info AS t2
    WHERE t1.name = t2.name;
  
  SELECT t1.name, t2.salary FROM employee t1, info t2
    WHERE t1.name = t2.name;

• Columns selected for output can be referred to in ORDER BY and GROUP BY clauses using column names, column aliases, or column positions. Column positions are integers and begin with 1:

  SELECT college, region, seed FROM tournament
    ORDER BY region, seed;
  
  SELECT college, region AS r, seed AS s FROM tournament
    ORDER BY r, s;
  
  SELECT college, region, seed FROM tournament
    ORDER BY 2, 3;

  To sort in reverse order, add the DESC (descending) keyword to the name of the column in the ORDER BY clause that you are sorting by. The default is ascending order; this can be specified explicitly using the ASC keyword.

  If ORDER BY occurs within a parenthesized query expression and also is applied in the outer query, the results are undefined and may change in a future MySQL version.

  Use of column positions is deprecated because the syntax has been removed from the SQL standard.

• MySQL extends the GROUP BY clause so that you can also specify ASC and DESC after columns named in the clause. However, this syntax is deprecated. To produce a given sort order, provide an ORDER BY clause.

• If you use GROUP BY, output rows are sorted according to the GROUP BY columns as if you had an ORDER BY for the same columns. To avoid the overhead of sorting that GROUP BY produces, add ORDER BY NULL:

  SELECT a, COUNT(b) FROM test_table GROUP BY a ORDER BY NULL;

  Relying on implicit GROUP BY sorting (that is, sorting in the absence of ASC or DESC designators) or explicit sorting for GROUP BY (that is, by using explicit ASC or DESC designators for GROUP BY columns) is deprecated. To produce a given sort order, provide an ORDER BY clause.

• When you use ORDER BY or GROUP BY to sort a column in a SELECT, the server sorts values using only the initial number of bytes indicated by the max_sort_length system variable.

• MySQL extends the use of GROUP BY to permit selecting fields that are not mentioned in the GROUP BY clause. If you are not getting the results that you expect from your query, please read the description of GROUP BY found in Section 12.19, “Aggregate Functions”.

• GROUP BY permits a WITH ROLLUP modifier. See Section 12.19.2, “GROUP BY Modifiers”.

• The HAVING clause, like the WHERE clause, specifies selection conditions. The WHERE clause specifies conditions on columns in the select list, but cannot refer to aggregate functions. The HAVING clause specifies conditions on groups, typically formed by the GROUP BY clause. The query result includes only groups satisfying the HAVING conditions. (If no GROUP BY is present, all rows implicitly form a single aggregate group.)

  The HAVING clause is applied nearly last, just before items are sent to the client, with no optimization. (LIMIT is applied after HAVING.)

  The SQL standard requires that HAVING must reference only columns in the GROUP BY clause or columns used in aggregate functions. However, MySQL supports an extension to this behavior, and permits HAVING to refer to columns in the SELECT list and columns in outer subqueries as well.

  If the HAVING clause refers to a column that is ambiguous, a warning occurs. In the following statement, col2 is ambiguous because it is used as both an alias and a column name:

  SELECT COUNT(col1) AS col2 FROM t GROUP BY col2 HAVING col2 = 2;

  Preference is given to standard SQL behavior, so if a HAVING column name is used both in GROUP BY and as an aliased column in the select column list, preference is given to the column in the GROUP BY column.

• Do not use HAVING for items that should be in the WHERE clause. For example, do not write the following:

  SELECT col_name FROM tbl_name HAVING col_name > 0;

  Write this instead:

  SELECT col_name FROM tbl_name WHERE col_name > 0;

• The HAVING clause can refer to aggregate functions, which the WHERE clause cannot:

  SELECT user, MAX(salary) FROM users
    GROUP BY user HAVING MAX(salary) > 10;

  (This did not work in some older versions of MySQL.)

• MySQL permits duplicate column names. That is, there can be more than one select_expr with the same name. This is an extension to standard SQL. Because MySQL also permits GROUP BY and HAVING to refer to select_expr values, this can result in an ambiguity:

  SELECT 12 AS a, a FROM t GROUP BY a;

  In that statement, both columns have the name a. To ensure that the correct column is used for grouping, use different names for each select_expr.

• MySQL resolves unqualified column or alias references in ORDER BY clauses by searching in the select_expr values, then in the columns of the tables in the FROM clause. For GROUP BY or HAVING clauses, it searches the FROM clause before searching in the select_expr values. (For GROUP BY and HAVING, this differs from the pre-MySQL 5.0 behavior that used the same rules as for ORDER BY.)

• The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must both be nonnegative integer constants, with these exceptions:

  • Within prepared statements, LIMIT parameters can be specified using ? placeholder markers.

  • Within stored programs, LIMIT parameters can be specified using integer-valued routine parameters or local variables.

  With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1):

  SELECT * FROM tbl LIMIT 5,10;  # Retrieve rows 6-15

  To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:

  SELECT * FROM tbl LIMIT 95,18446744073709551615;

  With one argument, the value specifies the number of rows to return from the beginning of the result set:

  SELECT * FROM tbl LIMIT 5;     # Retrieve first 5 rows

  In other words, LIMIT row_count is equivalent to LIMIT 0, row_count.

  For prepared statements, you can use placeholders. The following statements return one row from the tbl table:

  SET @a=1;
  PREPARE STMT FROM 'SELECT * FROM tbl LIMIT ?';
  EXECUTE STMT USING @a;

  The following statements return the second to sixth row from the tbl table:

  SET @skip=1; SET @numrows=5;
  PREPARE STMT FROM 'SELECT * FROM tbl LIMIT ?, ?';
  EXECUTE STMT USING @skip, @numrows;

  For compatibility with PostgreSQL, MySQL also supports the LIMIT row_count OFFSET offset syntax.

  If LIMIT occurs within a parenthesized query expression and also is applied in the outer query, the results are undefined and may change in a future MySQL version.

• A PROCEDURE clause names a procedure that should process the data in the result set. For an example, see Section 8.4.2.4, “Using PROCEDURE ANALYSE”, which describes ANALYSE, a procedure that can be used to obtain suggestions for optimal column data types that may help reduce table sizes.

  A PROCEDURE clause is not permitted in a UNION statement.

  Note

  PROCEDURE syntax is deprecated as of MySQL 5.7.18, and is removed in MySQL 8.0.

• The SELECT ... INTO form of SELECT enables the query result to be written to a file or stored in variables. For more information, see Section 13.2.9.1, “SELECT ... INTO Statement”.

• If you use FOR UPDATE with a storage engine that uses page or row locks, rows examined by the query are write-locked until the end of the current transaction. Using LOCK IN SHARE MODE sets a shared lock that permits other transactions to read the examined rows but not to update or delete them. See Section 14.7.2.4, “Locking Reads”.

  In addition, you cannot use FOR UPDATE as part of the SELECT in a statement such as CREATE TABLE new_table SELECT ... FROM old_table .... (If you attempt to do so, the statement is rejected with the error Can't update table 'old_table' while 'new_table' is being created.) This is a change in behavior from MySQL 5.5 and earlier, which permitted CREATE TABLE ... SELECT statements to make changes in tables other than the table being created.

Following the SELECT keyword, you can use a number of modifiers that affect the operation of the statement. HIGH_PRIORITY, STRAIGHT_JOIN, and modifiers beginning with SQL_ are MySQL extensions to standard SQL.

• The ALL and DISTINCT modifiers specify whether duplicate rows should be returned. ALL (the default) specifies that all matching rows should be returned, including duplicates. DISTINCT specifies removal of duplicate rows from the result set. It is an error to specify both modifiers. DISTINCTROW is a synonym for DISTINCT.

• HIGH_PRIORITY gives the SELECT higher priority than a statement that updates a table. You should use this only for queries that are very fast and must be done at once. A SELECT HIGH_PRIORITY query that is issued while the table is locked for reading runs even if there is an update statement waiting for the table to be free. This affects only storage engines that use only table-level locking (such as MyISAM, MEMORY, and MERGE).

  HIGH_PRIORITY cannot be used with SELECT statements that are part of a UNION.

• STRAIGHT_JOIN forces the optimizer to join the tables in the order in which they are listed in the FROM clause. You can use this to speed up a query if the optimizer joins the tables in nonoptimal order. STRAIGHT_JOIN also can be used in the table_references list. See Section 13.2.9.2, “JOIN Clause”.

  STRAIGHT_JOIN does not apply to any table that the optimizer treats as a const or system table. Such a table produces a single row, is read during the optimization phase of query execution, and references to its columns are replaced with the appropriate column values before query execution proceeds. These tables appear first in the query plan displayed by EXPLAIN. See Section 8.8.1, “Optimizing Queries with EXPLAIN”. This exception may not apply to const or system tables that are used on the NULL-complemented side of an outer join (that is, the right-side table of a LEFT JOIN or the left-side table of a RIGHT JOIN.

• SQL_BIG_RESULT or SQL_SMALL_RESULT can be used with GROUP BY or DISTINCT to tell the optimizer that the result set has many rows or is small, respectively. For SQL_BIG_RESULT, MySQL directly uses disk-based temporary tables if they are created, and prefers sorting to using a temporary table with a key on the GROUP BY elements. For SQL_SMALL_RESULT, MySQL uses in-memory temporary tables to store the resulting table instead of using sorting. This should not normally be needed.

• SQL_BUFFER_RESULT forces the result to be put into a temporary table. This helps MySQL free the table locks early and helps in cases where it takes a long time to send the result set to the client. This modifier can be used only for top-level SELECT statements, not for subqueries or following UNION.

• SQL_CALC_FOUND_ROWS tells MySQL to calculate how many rows there would be in the result set, disregarding any LIMIT clause. The number of rows can then be retrieved with SELECT FOUND_ROWS(). See Section 12.15, “Information Functions”.

• The SQL_CACHE and SQL_NO_CACHE modifiers affect caching of query results in the query cache (see Section 8.10.3, “The MySQL Query Cache”). SQL_CACHE tells MySQL to store the result in the query cache if it is cacheable and the value of the query_cache_type system variable is 2 or DEMAND. With SQL_NO_CACHE, the server does not use the query cache. It neither checks the query cache to see whether the result is already cached, nor does it cache the query result.

  These two modifiers are mutually exclusive and an error occurs if they are both specified. Also, these modifiers are not permitted in subqueries (including subqueries in the FROM clause), and SELECT statements in unions other than the first SELECT.

  For views, SQL_NO_CACHE applies if it appears in any SELECT in the query. For a cacheable query, SQL_CACHE applies if it appears in the first SELECT of a view referred to by the query.

  Note

  The query cache is deprecated as of MySQL 5.7.20, and is removed in MySQL 8.0. Deprecation includes SQL_CACHE and SQL_NO_CACHE.

A SELECT from a partitioned table using a storage engine such as MyISAM that employs table-level locks locks only those partitions containing rows that match the SELECT statement WHERE clause. (This does not occur with storage engines such as InnoDB that employ row-level locking.) For more information, see Section 22.6.4, “Partitioning and Locking”.