MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6
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
unqualified name t1
:
CREATE TABLE t1 (i INT);
Because 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
db1.t1
:
CREATE TABLE db1.t1 (i INT);
Because 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_name
and
tbl_name . col_name
are
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
`my-table`.`my-column`
, not
`my-table.my-column`
.
A reserved word that follows a period in a qualified name must be an identifier, so in that context it need not be quoted.
The syntax
means the table .tbl_name
tbl_name
in the
default database.
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
statements:
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_name |
Column col_name from whichever table used in
the statement contains a column of that name |
tbl_name.col_name |
Column col_name from table
tbl_name of the default
database |
db_name.tbl_name.col_name |
Column col_name from table
tbl_name of the database
db_name |
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
statements:
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
that column c1
occurs only in table
t1
, c2
only in
t2
, and c
in both
t1
and t2
. Any unqualified
reference to c
is ambiguous in a statement
that refers to both tables and must be qualified as
t1.c
or 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 t
in
database 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
column c1
occurs only in table
db1.t
, c2
only in
db2.t
, and c
in both
db1.t
and db2.t
. In this
case, c
is ambiguous and must be qualified
but c1
and 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;