postgresql中null对!=的影响

今天开发问我，为什么!=操作不能获取值为null记录：

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abce=# select id, abce-#         a_no, abce-#         a_code, abce-#         a_number, abce-#         date_receive, abce-#         bank_a, abce-#         status abce-# from abce-#         erp_a_examine abce-# where abce-#         ( a_number = '1023' and a_code = '231174' and status != '已作废' );  id  | a_no | a_code | a_number | date_receive | bank_a | status -----+------+--------+----------+--------------+--------+-------- (0 rows)   abce=# select id, abce-#         a_no, abce-#         a_code, abce-#         a_number, abce-#         date_receive, abce-#         bank_a, abce-#         status abce-# from abce-#         erp_a_examine abce-# where abce-#         ( a_number = '1023' and a_code = '231174' );  id  | a_no | a_code | a_number | date_receive | bank_a | status -----+------+--------+----------+--------------+--------+--------  333 | 1023 | 231174 | 12155    | 2010-03-02   |        | (1 row)   abce=#

　　

只是因为null是非safe的操作。