where子句中的參數基礎上組嵌套查詢語句

Question

我想形成一個SQL查詢類似以下，但無法弄清楚如何使它發揮作用：

SELECT col_1 AS a, 
     col_2 AS b, 
     (SELECT sum(col_3) 
     FROM table2 
     WHERE col_1 = a 
      AND col_2 = b) AS c 
FROM table1 
GROUP BY col_1, col_2; 

Answer 1

列別名不可用的子查詢。

你必須直接引用列：

SELECT col_1 AS a, 
     col_2 AS b, 
     (SELECT sum(col_3) 
     FROM table2 
     WHERE col_1 = table1.col_1 
      AND col_2 = table1.col_2) AS c 
FROM table1 
GROUP BY col_1, col_2;