在mysql中，字符串和数字比，字符串会被转成数字0。下面我演示：
mysql> create table t (a int primary key, b int,c int, d int, index idx_b_c_d (b,c,d)); Query OK, 0 rows affected (0.14 sec) mysql> insert into t values (0,0,0,0),(1,1,1,1),(2,2,2,2),(3,3,3,3); Query OK, 4 rows affected (0.00 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql> select * from t; +---+------+------+------+ | a | b | c | d | +---+------+------+------+ | 0 | 0 | 0 | 0 | | 1 | 1 | 1 | 1 | | 2 | 2 | 2 | 2 | | 3 | 3 | 3 | 3 | +---+------+------+------+ 4 rows in set (0.00 sec)
下面演示奇怪现象：
mysql> select * from t where a=1; +---+------+------+------+ | a | b | c | d | +---+------+------+------+ | 1 | 1 | 1 | 1 | +---+------+------+------+ 1 row in set (0.00 sec) mysql> select * from t where a='t'; +---+------+------+------+ | a | b | c | d | +---+------+------+------+ | 0 | 0 | 0 | 0 | +---+------+------+------+ 1 row in set, 1 warning (0.00 sec) mysql> select * from t where a='ttt'; +---+------+------+------+ | a | b | c | d | +---+------+------+------+ | 0 | 0 | 0 | 0 | +---+------+------+------+ 1 row in set, 1 warning (0.00 sec) mysql> select * from t where a='bbb'; +---+------+------+------+ | a | b | c | d | +---+------+------+------+ | 0 | 0 | 0 | 0 | +---+------+------+------+ 1 row in set, 1 warning (0.00 sec)
可以看到，where a='bbb' 能查出a=0的行，这就是因为mysql把字符串bbb转成了数字0。 通过下面可以验证，1表示为真，0表示为假：
mysql> select 'ttt'=0; +---------+ | 'ttt'=0 | +---------+ | 1 | +---------+ 1 row in set, 1 warning (0.01 sec) mysql> select 'ttt'=1; +---------+ | 'ttt'=1 | +---------+ | 0 | +---------+ 1 row in set, 1 warning (0.00 sec) mysql> select 'bbb'=1; +---------+ | 'bbb'=1 | +---------+ | 0 | +---------+ 1 row in set, 1 warning (0.00 sec)
