为多个选择计数（*）表设置列名称

Question

如何为波纹管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，我把括号整个表达式。这不是必需的，但它使代码更具可读性。我也修复了子查询。

如果你想这个多次运行，那么相关子查询使得它更易于管理的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，修改查询，你只需要在一个地方更改值。

Answer 2

使用为关键字

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))