Question

如何为bellow查询的输出设置列名？

select 
(select count(*) from t_table1 id = 1)
+
(select count(*) from t_table2 id = 1)
+
(select count(*) from t_table3 id = 1)

Answer 1

使用as：

select ( (select count(*) from t_table1 where id = 1) +
         (select count(*) from t_table2 where id = 1) +
         (select count(*) from t_table3 where id = 1)
       ) as col

请注意，我将整个表达式放在括号中。这不是必需的，但它使代码更具可读性。我还修复了子查询。

如果要多次运行，则相关子查询可以更轻松地管理ID：

select ( (select count(*) from t_table1 t where t.id = x.id) +
         (select count(*) from t_table2 t where t.id = x.id) +
         (select count(*) from t_table3 t where t.id = x.id)
       ) as col
from (select 1 as id) x;

然后，要修改查询，您只需要在一个地方更改值。

Answer 2

使用作为关键字

select (select count(*) from t_table1 id = 1)+
(select count(*) from t_table2 id = 1)+
(select count(*) from t_table3 id = 1) as result

Answer 3

select sum(count_tab) as col_name from(
(select count(*) as count_tab from t_table1 id = 1)
union all
(select count(*) from t_table2 id = 1)
union all
(select count(*) from t_table3 id = 1))

为多个选择计数（*）表设置列名

3 个答案: