Question

Mysql, I am using SQL_CALC_FOUND_ROWS to get the total number of possible records.
How do I use it on UNION?
The only thing that works for me right now, which seems somewhat off is:

SELECT COUNT(*)
FROM(SELECT * FROM t1 UNION SELECT * FROM t2) A;

or

SELECT SQL_CALC_FOUND_ROWS *
FROM(SELECT * FROM t1 UNION SELECT * FROM t2) A;

Answer 1

From the FOUND_ROWS() documentation:

The use of SQL_CALC_FOUND_ROWS and
FOUND_ROWS() is more complex for UNION
statements than for simple SELECT
statements, because LIMIT may occur at
multiple places in a UNION. It may be
applied to individual SELECT
statements in the UNION, or global to
the UNION result as a whole.

The intent of SQL_CALC_FOUND_ROWS for
UNION is that it should return the row
count that would be returned without a
global LIMIT. The conditions for use
of SQL_CALC_FOUND_ROWS with UNION are:

• The SQL_CALC_FOUND_ROWS keyword must
  appear in the first SELECT of the
  UNION.

• The value of FOUND_ROWS() is exact
  only if UNION ALL is used. If UNION
  without ALL is used, duplicate removal
  occurs and the value of FOUND_ROWS()
  is only approximate.

• If no LIMIT is present in the UNION,
  SQL_CALC_FOUND_ROWS is ignored and
  returns the number of rows in the
  temporary table that is created to
  process the UNION.