Mysql從2個表中計數相同的值作爲2個不同的計數

Question

我想計算一個存在於2個不同表中的值。但應顯示分隔這樣的：

我當然可以seperately指望它。但我想在一個查詢中得到一個結果。感謝您的幫助

SELECT `X1`, COUNT(`X1`) AS Sales FROM `table1` GROUP BY `X1` 

SELECT `X1`, COUNT(`X1`) AS Purchases FROM `table2` GROUP BY `X1` 

Answer 1

這是一種痛苦，因爲每個表中的x1值的集合可能不一樣。

下面是使用union all和group by一種方法：

select x1, sum(sales) as sales, sum(purchases) as purchases 
from ((select x1, count(*) as sales, 0 as purchases 
     from table1 
     group by x1 
    ) union all 
     (select x2, 0, count(*) 
     from table2 
     group by x1 
    ) 
    ) t12 
group by x1; 

Answer 2

使用子查詢從表中的一個獲得計數，然後再加入與其他表。

SELECT t1.x1, COUNT(*) AS sales, t2.purchases 
FROM table1 AS t1 
JOIN (SELECT x1, COUNT(*) AS purchases 
     FROM table2 
     GROUP BY x1) AS t2 
ON t1.x1 = t2.x1 
GROUP BY t1.x1 

在兩個子查詢中進行分組可能確實更高效。

SELECT t1.x1, t1.sales, t2.purchases 
FROM (SELECT x1, COUNT(*) AS sales 
     FROM table1 
     GROUP BY x1) AS t1 
JOIN (SELECT x1, COUNT(*) AS purchases 
     FROM table2 
     GROUP BY x1) AS t2 
ON t1.x1 = t2.x1 

但是，如果有任何x1值不在兩個表中，這兩個查詢將離開他們。理想的解決方案是FULL OUTER JOIN，但MySQL不具備此操作。有關解決方法，請參見Full Outer Join in MySQL。 Gordon Linoff的答案沒有這個問題。或者，如果您有另一個列出所有x1值的表，則可以將其用作主表，並使用LEFT JOIN以上兩個子查詢。