If the value is 0, all projects are displayed ~ View source1 mysql> select * from login where username = 0; 2 + ----------- + ---------- + 3 | username | password | 4 + ----------- + ---------- + 5 | administrator | password | 6 | aaa | ccc | 7 + ----------- + ---------- + 8 2 rows in set (0.11 sec) while 'A' + 'B' is equal to 0 view source1 mysql> select 'A' + 'B '; 2 + --------- + 3 | 'A' + 'B' | 4 + --------- + 5 | 0 | 6 + --------- + view source1 mysql> show warnings; 2 + --------- + ------ + upper + 3 | Level | Code | Message | 4 + --------- + ------ + ----------------------------------------- + 5 | Warning | 1292 | Truncated incorrect DOUBLE value: 'A' | 6 | Warning | 1292 | Truncated incorrect DOUBLE value: 'B' | 7 + --------- + ------ + --------------------------------------- + 8 2 rows in set (0.00 sec) Details: http://www.bkjia.com/Article/201304/204266.htmlhttp://www.bkjia.com/Article/201304/205894.html