如何重構這個MySQL代碼？

Question

SELECT AVG(`col5`) 
    FROM `table1` 
    WHERE `id` NOT IN (
     SELECT `id` FROM `table2` 
     WHERE `col4` = 5 
    ) 
    group by `col2` having sum(`col3`) > 0 
UNION 
    SELECT MAX(`col5`) 
    FROM `table1` 
    WHERE `id` NOT IN (
     SELECT `id` FROM `table2` 
     WHERE `col4` = 5 
    ) 
    group by `col2` having sum(`col3`) = 0 

出於可讀性和性能的原因，我認爲這個代碼可以重構。但是如何？

EDITIONS

1. 除去外選擇

2. 作出的第一選擇返回的總和，而第二個返回另一個值

3. 取代SUM通過AVG

Answer 1

SELECT * 
FROM table1 t1 
left outer join table2 t2 on t1.id = t2.id and t2.col4 = 5 
where t2.id is null 
group by t1.col2 
having sum(col3) >= 0 

外部選擇缺少FROM子句並且沒有添加任何內容，因此我將其刪除。與LEFT OUTER JOIN方法相比，NOT IN效率低下，因此我將其替換。通過使用>=，兩個UNION很容易組合成一個。

更新： 注意使用UNION ALL而非UNION。我不想認爲你想刪除重複項，並且它會以這種方式更快地執行。

SELECT AVG(t1.col5) 
FROM table1 t1 
left outer join table2 t2 on t1.id = t2.id and t2.col4 = 5 
where t2.id is null 
group by t1.col2 
having sum(t1.col3) > 0 
UNION ALL 
SELECT MAX(t1.col5) 
FROM table1 t1 
left outer join table2 t2 on t1.id = t2.id and t2.col4 = 5 
where t2.id is null 
group by t1.col2 
having sum(t1.col3) = 0 

Answer 2

SELECT t1.* FROM table1 t1 LEFT OUTER JOIN table2 t2 ON t1.id = t2.id 
WHERE t2.col4 <> 5 AND SUM(t1.col3) > 0 GROUP BY t1.col2 

Answer 3

我猜，你想這樣的：

SELECT * FROM `table` 
WHERE col2 IN 
(SELECT col2 
    FROM `table1` 
    WHERE `id` NOT IN ( 
     SELECT `id` FROM `table2` 
     WHERE `col4` = 5 
    ) 
    group by `col2` having sum(`col3`) >= 0 
) 

使用GROUP BY，你應該只返回屬於GROUP BY子句中指定或包括聚合函數列。因此，這裏的內部SELECT獲得col2值，其中總和大於或等於零，然後外部SELECT抓取這些值的整個行。

Answer 4

SELECT 
    IF(SUM(`table1`.`col3`) > 0, AVG(`table1`.`col5`), MAX(`table1`.`col5`)) 
FROM `table1` 
    LEFT JOIN `table2` ON `table2`.`id` = `table1`.`id` AND `table2`.`col4` = 5 
WHERE `table2`.`id` IS NULL 
GROUP BY `table1`.`col2` 
HAVING SUM(`table1`.`col3`) >= 0 

另外*被認爲是有害的。如果您想使查詢向前兼容未來對數據庫模型的可能更改，請按名稱指定列。