SQL-如何在JOIN条件下执行IF / ELSE

Question

我知道如何在联接条件下执行IF / ELSE，但是有没有一种方法可以基于ELSE一起对字段进行完全不同的评估？

SELECT o.id, s.id
FROM orders o
RIGHT JOIN subjects s ON s.name = 'Blah'
                     AND s.group_id = IF(o.service = 'Law Services', 2, ????)

在上面，我不知道要放什么????

我想说的是（伪代码）：

IF o.service = 'Law Services'

    JOIN subjects s ON s.name = 'Blah' AND s.group_id = 2

ELSE

    JOIN subjects s ON s.name = 'Blah'

基本上，我只想在s.group_id = 2添加JOIN条件。

Answer 1

您可以编写条件而不使用if ：

SELECT o.id, s.id
FROM orders o RIGHT JOIN
     subjects s
     ON s.name = 'Blah' AND
        (o.service <> 'Law Services' or s.group_id = 2);

Answer 2

这可能值得一试：

SELECT o.id, s.id
FROM orders o
RIGHT JOIN subjects s 
   ON s.name = 'Blah'
     AND s.group_id = (CASE WHEN o.service = 'Law Services' THEN 2 ELSE s.group_id END)

Answer 3

SELECT o.id, s.id
FROM orders o
RIGHT JOIN subjects s ON s.name = 'Blah'
                     AND s.group_id = IF(o.service = 'Law Services', 2, s.group_id)