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 13.2.15.5, “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 works. If the subquery returns more than one row, error 1242 occurs. 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 common table expression or derived table to work around this. See Section 13.2.15.12, “Restrictions on Subqueries” . In MySQL 8.0.19 and later, all of the errors described in this section also apply when using TABLE in subqueries. 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.