9.2.1. Identifier Qualifiers
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:
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,
whereas `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 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.col_name and
db2.t.col_name.
A word that follows a period in a qualified name must be an
identifier, so it is not necessary to quote it, even if it is a
reserved word.
The syntax .tbl_name means the table
tbl_name in the default database.
This syntax is accepted for ODBC compatibility because some ODBC
programs prefix table names with a
‘.’ character.
