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MySQL allows names that consist of a single identifier or multiple identifiers. The components of a multiple-part name should be separated by period (`.') characters. The initial parts of a multiple-part name act as qualifiers that affect the context within which the final identifier is interpreted.
In MySQL you can refer to a column using any of the following forms:
Column reference | Meaning |
col_name | Column col_name |
from whichever table used in the query contains a column of that name.
tbl_name.col_name | Column col_name from table |
db_name.tbl_name.col_name | Column col_name from table |
If any components of a multiple-part name require quoting, quote them
individually rather than quoting the name as a whole. For example,
`my-table`.`my-column`
is legal, but `my-table.my-column`
is
not.
You need not specify a tbl_name
or db_name.tbl_name
prefix for
a column reference in a statement unless the reference would be ambiguous.
Suppose tables t1
and t2
each contain a column
c
, and you retrieve c
in a SELECT
statement that uses
both t1
and t2
. In this case, c
is ambiguous because it
is not unique among the tables used in the statement. You must qualify it
with a table name as t1.c
or t2.c
to indicate which table you
mean. Similarly, to retrieve from a table t
in database db1
and from a table t
in database db2
in the same statement,
you must refer to columns in those tables as db1.t.col_name
and db2.t.col_name
.
The syntax .tbl_name
means the table tbl_name
in the current
database. This syntax is accepted for ODBC compatibility, because some ODBC
programs prefix table names with a `.' character.
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