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SELECT @a:=0; SELECT @a FROM (SELECT 1) t WHERE @a:=1 AND @a = 1;

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SELECT @a:=1; SELECT @a FROM (SELECT 1) t WHERE @a:=1 AND @a = 1;

�� SELECT �� Empty ��

�� SELECT �� 1 ��

�� WHERE �� True ��

�� WHERE �� 0 �� SELECT @a:=1 ��

True ��

SELECT @a:=1; SELECT @a FROM (SELECT 1) t WHERE @a:=0 AND @a = 1;

�� WHERE �� @a:=2 ��

mysql> SELECT @a:=1; SELECT @a FROM (SELECT 1) t WHERE @a:=2 AND @a = 1;

+-------+

| @a:=1 |

+-------+

| 1 |

+-------+

1 row in set (0.00 sec)

+------+

| @a |

+------+

| 1 |

+------+

1 row in set (0.00 sec)

��@a:=2 �� := 0 ��w��

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mysql Ver 14.14 Distrib 5.6.25, for Linux (x86_64)