Question

我需要选择两个但具有不同条件的字段，我试过这个：

SELECT 
(select my_field from my_table where another_field=1) as column_one,
(select my_field from my_table where another_field=2) as column_two;

但是我收到了这个错误：

Subquery returns more than 1 row

有没有办法使这项工作？

Answer 1

它可以返回超过1行，因此您应该使用限制1

SELECT (select my_field from my_table where another_field=1 limit 1) as column_one,(select my_field from my_table where another_field=2 limit 1) as column_two;

查询应该是这样的

 select t1.my_field as column_one,t2.my_field as column_two
from my_table as t1
left join (select my_field from my_table where another_field=2)  t2 on t1.key_field=t2.key_field
where t1.another_field=1;

Answer 2

如错误消息所述，其中一个子查询返回多行，因此mysql无法在单行的单个列中输出该子查询的结果。

确保这两个查询仅返回一行：

select my_field from my_table where another_field=1
select my_field from my_table where another_field=2

Answer 3

在目前的形式中，您可以通过添加limit子句来使其工作：

SELECT 
(select my_field from my_table where another_field=1 limit 1) as column_one,
(select my_field from my_table where another_field=2 limit 1) as column_two;

但是，从逻辑上讲它没有任何意义，因为没有附带limit的{{1}}子句不能保证每次查询运行时都有特定的结果。

选择具有不同条件的字段两次

3 个答案: