Object names may be unqualified or qualified. An unqualified name is permitted in contexts where interpretation of the name is unambiguous. A qualified name includes at least one qualifier to clarify the interpretive context by overriding a default context or providing missing context.
For example, this statement creates a table using the
CREATE TABLE t1 (i INT);
t1 includes no qualifier to specify a
database, the statement creates the table in the default
database. If there is no default database, an error occurs.
This statement creates a table using the qualified name
CREATE TABLE db1.t1 (i INT);
db1.t1 includes a database qualifier
db1, the statement creates
t1 in the database named
db1, regardless of the default database. The
qualifier must be specified if there is no
default database. The qualifier may be
specified if there is a default database, to specify a database
different from the default, or to make the database explicit if
the default is the same as the one specified.
Qualifiers have these characteristics:
An unqualified name consists of a single identifier. A qualified name consists of multiple identifiers.
The components of a multiple-part name must be separated by period (
.) characters. The initial parts of a multiple-part name act as qualifiers that affect the context within which to interpret the final identifier.
The qualifier character is a separate token and need not be contiguous with the associated identifiers. For example,
tbl_name . col_nameare equivalent.
If any components of a multiple-part name require quoting, quote them individually rather than quoting the name as a whole. For example, write
A reserved word that follows a period in a qualified name must be an identifier, so in that context it need not be quoted.
means the table
tbl_namein the default database.Note
This syntax is deprecated as of MySQL 5.7.20; expect it to be removed in a future version of MySQL.
The permitted qualifiers for object names depend on the object type:
A database name is fully qualified and takes no qualifier:
CREATE DATABASE db1;
A table, view, or stored program name may be given a database-name qualifier. Examples of unqualified and qualified names in
CREATE TABLE mytable ...; CREATE VIEW myview ...; CREATE PROCEDURE myproc ...; CREATE FUNCTION myfunc ...; CREATE EVENT myevent ...; CREATE TABLE mydb.mytable ...; CREATE VIEW mydb.myview ...; CREATE PROCEDURE mydb.myproc ...; CREATE FUNCTION mydb.myfunc ...; CREATE EVENT mydb.myevent ...;
A trigger is associated with a table, so any qualifier applies to the table name:
CREATE TRIGGER mytrigger ... ON mytable ...; CREATE TRIGGER mytrigger ... ON mydb.mytable ...;
A column name may be given multiple qualifiers to indicate context in statements that reference it, as shown in the following table.
Column Reference Meaning
col_namefrom whichever table used in the statement contains a column of that name
tbl_nameof the default database
tbl_nameof the database
In other words, a column name may be given a table-name qualifier, which itself may be given a database-name qualifier. Examples of unqualified and qualified column references in
SELECT c1 FROM mytable WHERE c2 > 100; SELECT mytable.c1 FROM mytable WHERE mytable.c2 > 100; SELECT mydb.mytable.c1 FROM mydb.mytable WHERE mydb.mytable.c2 > 100;
You need not specify a qualifier for an object reference in a
statement unless the unqualified reference is ambiguous. Suppose
c1 occurs only in table
c2 only in
c in both
t2. Any unqualified
c is ambiguous in a statement
that refers to both tables and must be qualified as
t2.c to indicate
which table you mean:
SELECT c1, c2, t1.c FROM t1 INNER JOIN t2 WHERE t2.c > 100;
Similarly, to retrieve from a table
db1 and from a table
t in database
db2 in the
same statement, you must qualify the table references: For
references to columns in those tables, qualifiers are required
only for column names that appear in both tables. Suppose that
c1 occurs only in table
c2 only in
c in both
db2.t. In this
c is ambiguous and must be qualified
c2 need not be:
SELECT c1, c2, db1.t.c FROM db1.t INNER JOIN db2.t WHERE db2.t.c > 100;
Table aliases enable qualified column references to be written more simply:
SELECT c1, c2, t1.c FROM db1.t AS t1 INNER JOIN db2.t AS t2 WHERE t2.c > 100;