爲多個選擇計數（*）表設置列名稱

Question

如何爲波紋管查詢的輸出設置列名？

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 

請注意，我把括號整個表達式。這不是必需的，但它使代碼更具可讀性。我也修復了子查詢。

如果你想這個多次運行，那麼相關子查詢使得它更易於管理的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; 

然後，修改查詢，你只需要在一個地方更改值。

Answer 2

使用爲關鍵字

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))