COALESCE函数从值列表中返回第一个非NULL的值,当遇到NULL值时将其替换为0。 coalesce(str1,str2....);
e.g. 需要在表中查出所有比'WARD'提成(COMM)低的员工,提成为NULL的员工也包括在内。 (个人意见,如果数据库提成字段默认值不是为0值的话肯定是开发那个的错)。
select ename,comm from emp where coalesce(comm,0) < (select comm from emp whrer ename ="WARD");
结果:
+--------+------+ | ename | comm | +--------+------+ | SMITH | NULL | | ALLEN | 300 | | JONES | NULL | | BLAKE | NULL | | CLARK | NULL | | SCOTT | NULL | | KING | NULL | | TURNER | 0 | | JAMES | NULL | | MILLER | NULL | | ADAMS | NULL | | FORD | NULL | +--------+------+ 12 rows in set
返回非NULL值:
select ename, comm,coalesce(comm,0) from emp where coalesce(comm,0) < (select comm from emp where ename = 'WARD');
+--------+------+------------------+ | ename | comm | coalesce(comm,0) | +--------+------+------------------+ | SMITH | NULL | 0 | | ALLEN | 300 | 300 | | JONES | NULL | 0 | | BLAKE | NULL | 0 | | CLARK | NULL | 0 | | SCOTT | NULL | 0 | | KING | NULL | 0 | | TURNER | 0 | 0 | | JAMES | NULL | 0 | | MILLER | NULL | 0 | | ADAMS | NULL | 0 | | FORD | NULL | 0 | +--------+------+------------------+ 12 rows in set