When both operands are non-NULL, the result is 1 if any operand is non-zero, and 0 otherwise.
With a NULL operand, the result is 1 if the other operand is non-zero, and NULL otherwise.
If both operands are NULL, the result is NULL.
mysql>mysql> SELECT 1 || 1;+--------+| 1 || 1 |+--------+| 1 |+--------+1 row in set (0.00 sec)mysql> SELECT 1 || 0;+--------+| 1 || 0 |+--------+| 1 |+--------+1 row in set (0.00 sec)mysql> SELECT 0 || 0;+--------+| 0 || 0 |+--------+| 0 |+--------+1 row in set (0.00 sec)
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