MySQL allows names that consist of a single identifier or
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 the final identifier is interpreted.
In MySQL, you can refer to a table column using any of the following forms.
Column Reference | Meaning |
col_name |
The column col_name from whichever table used
in the statement contains a column of that name. |
tbl_name.col_name |
The column col_name from table
tbl_name of the default
database. |
db_name.tbl_name.col_name |
The column col_name from table
tbl_name of the database
db_name . |
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.
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 that 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.
and
col_name
db2.t.
.
col_name
The syntax
means
the table .tbl_name
tbl_name
in the default
database. This syntax is accepted for ODBC compatibility because
some ODBC programs prefix table names with a
“.
” character.
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