Oracle 5.0, Reference Manual

Page: 1126 / 3232

Subquery Syntax
1106
This means that MySQL does not support statements of the following form:
SELECT * FROM t1 WHERE s1 IN (SELECT s2 FROM t2 ORDER BY s1 LIMIT 1)
• Incorrect number of columns from subquery:
ERROR 1241 (ER_OPERAND_COL)
SQLSTATE = 21000
Message = "Operand should contain 1 column(s)"
This error occurs in cases like this:
SELECT (SELECT column1, column2 FROM t2) FROM t1;
You may use a subquery that returns multiple columns, if the purpose is row comparison. In other
contexts, the subquery must be a scalar operand. See
Section 13.2.9.5, “Row Subqueries” .
• Incorrect number of rows from subquery:
ERROR 1242 (ER_SUBSELECT_NO_1_ROW)
SQLSTATE = 21000
Message = "Subquery returns more than 1 row"
This error occurs for statements where the subquery must return at most one row but returns multiple
rows. Consider the following example:
SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);
If
SELECT column1 FROM t2
returns just one row, the previous query will work. If the subquery
returns more than one row, error 1242 will occur. In that case, the query should be rewritten as:
SELECT * FROM t1 WHERE column1 = ANY (SELECT column1 FROM t2);
• Incorrectly used table in subquery:
Error 1093 (ER_UPDATE_TABLE_USED)
SQLSTATE = HY000
Message = "You can't specify target table 'x'
for update in FROM clause"
This error occurs in cases such as the following, which attempts to modify a table and select from the
same table in the subquery:
UPDATE t1 SET column2 = (SELECT MAX(column1) FROM t1);
You can use a subquery for assignment within an
UPDATE
statement because subqueries are legal
in
UPDATE
and
DELETE
statements as well as in
SELECT
statements. However, you cannot use the
same table (in this case, table
t1
) for both the subquery
FROM
clause and the update target.
For transactional storage engines, the failure of a subquery causes the entire statement to fail. For
nontransactional storage engines, data modifications made before the error was encountered are
preserved.
13.2.9.10. Optimizing Subqueries
Development is ongoing, so no optimization tip is reliable for the long term. The following list provides
some interesting tricks that you might want to play with:
• Use subquery clauses that affect the number or order of the rows in the subquery. For example:
SELECT * FROM t1 WHERE t1.column1 IN
(SELECT column1 FROM t2 ORDER BY column1);
SELECT * FROM t1 WHERE t1.column1 IN
(SELECT DISTINCT column1 FROM t2);
SELECT * FROM t1 WHERE EXISTS