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 comparison. See Section 13.2.8.5, “Row Subqueries”. However, in other contexts, the subquery must be a scalar operand.

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 returns more than one row. 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:

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's FROM clause and the update target.