create table testTable(
id int,
val int
);
insert into testTable values(1, null);
insert into testTable values(null, 1);
insert into testTable values(1, 1);
select count(id), count(val), count(*), count(1) from testTable;
id int,
val int
);
insert into testTable values(1, null);
insert into testTable values(null, 1);
insert into testTable values(1, 1);
select count(id), count(val), count(*), count(1) from testTable;
可见 count 函数 对字段值count操作时 值为null的没有计算。
select sum(val),sum(id) from testTable;
聚合或其他 SET 操作消除了 Null 值。