理解NULL如何影响IN和EXITS语句_MySQL
select 'true' from dual where 1 = null;
select 'true' from dual where 1 != null;
只有IS NULL才能返回true,并返回一行:
select 'true' from dual where 1 is null;
select 'true' from dual where null is null;
当你选择使用IN,你将会告诉SQL选择一个值并与其它每一值相比较。如果NULL值存在,将不会返回一行,即使两个都为NULL。
select 'true' from dual where null in (null);
select 'true' from dual where (null,null) in ((null,null));
select 'true' from dual where (1,null) in ((1,null));
一个IN语句在功能上相当于= ANY语句:
select 'true' from dual where null = ANY (null);
select 'true' from dual where (null,null) = ANY ((null,null));
select 'true' from dual where (1,null) = ANY ((1,null));
当你使用一个EXISTS等效形式的语句,SQL将会计算所有行,并忽略子查询中的值。
select 'true' from dual where exists (select null from dual);
select 'true' from dual where exists (select 0 from dual where null is null);
IN和EXISTS在逻辑上是相同的。IN语句比较由子查询返回的值,并在输出查询中过滤某些行。EXISTS语句比较行的值,并在子查询中过滤某些行。对于NULL值的情况,行的结果是相同的。
selectename from emp where empno in (select mgr from emp);
selectename from emp e where exists (select 0 from emp where mgr = e.empno);
然而当逻辑被逆向使用,即NOT IN 及NOT EXISTS时,问题就会产生:
selectename from emp where empno not in (select mgr from emp);
selectename from emp e where not exists (select 0 from emp where mgr =
e.empno);
NOT IN语句实质上等同于使用=比较每一值,如果测试为FALSE或者NULL,结果为比较失败。例如:
select 'true' from dual where 1 not in (null,2);
select 'true' from dual where 1 != null and 1 != 2;
select 'true' from dual where (1,2) not in ((2,3),(2,null));
select 'true' from dual where (1,null) not in ((1,2),(2,3));
这些查询不会返回任何一行。第二个查询语句更为明显,即1 != null,所以整个WHERE都为false。然而这些查询语句可变为:
select 'true' from dual where 1 not in (2,3);
select 'true' from dual where 1 != 2 and 1 != 3;
你也可以使用NOT IN查询,只要你保证返回的值不会出现NULL值:
selectename from emp where empno not in (select mgr from emp where mgr is not
null);
selectename from emp where empno not in (select nvl(mgr,0) from emp);
通过理解IN,EXISTS, NOT IN,以及NOT EXISTS之间的差别,当NULL出现在任一子查询中时,你可以避免一些常见的问题。