mysql - 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

使用作为:

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。

如果你想多次运行这个，那么相关的子查询可以更容易地管理 ids:

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，您只需要更改一个地方的值。