mysql-表的兩列和另一列的計數

Question

我希望這個查詢能合而為一：

select count(first_column) as first from table1 where user_id = $id
select count(second_column) as second from table1 where user_id = $id
select count(first_column) as third from table2 where user_id = $id

我有這個：

select COUNT(table1.first_column) AS first, 
       COUNT(table1.second_column) AS second, 
       COUNT(table2.first_column) AS third 
from table2 inner join
        table2 on table1.user_id = table2.user_id 
where table1.user_id = 1;

但它為table2返回錯誤的值。 我怎樣才能解決這個問題？

Answer 1

一種簡單的方法是使用子查詢：

select (select count(first_column) as first from table1 where user_id = $id) as first,
       (select count(second_column) as second from table1 where user_id = $id) as second,
       (sselect count(first_column) as third from table2 where user_id = $id) as third;

因為前兩個來自同一張表，所以您可以考慮：

select count(t1.first_column), count(t1.second_column), t2.third_column
from table1 t1 cross join
     (select count(third_column as third from table2) t2
where t1.user_id = $id;

這將在MySQL中工作。 在其他數據庫中，第三列需要一個聚合函數。