Question

select colA, colB from car
where car.id in (select id from make) and 
car.id in (select id from model);

上面的查询工作原理完全正确，比如说有一种情况是make表没有被任何东西填充。只是一张空桌子。有没有办法让连接不在那张桌子上？

基本上，如果表有1行或更多行，请应用where条件。否则，忽略并且不限制它。

有没有办法在左连接上获得相同的结果？

修改

结果算法：

从原始表车中选择东西。
带走任何不在表格中的内容
除去表格模型中的任何内容....
如果model2有任何内容，则会删除表model2中的任何内容....

模型2的Thisnk是我不想要的东西的表，并且模型和制作我想要的东西的表。

Answer 1

SELECT colA, colB
FROM car
WHERE ((SELECT COUNT(*) FROM make) = 0) OR id IN (SELECT id FROM make))
  AND id IN (SELECT id from model)

左键加入：

SELECT DISTINCT colA, colB
FROM car
JOIN (SELECT COUNT(*) c FROM make) mcount
LEFT JOIN make ON car.id = make.id
JOIN model ON car.id = model.id
WHERE mcount.c = 0 OR make.id IS NOT NULL

使用OR通常可以防止使用索引，因此使用UNION可能更好：

SELECT distinct colA, colB
FROM car
JOIN make on car.id = make.id
JOIN model on car.id = model.id

UNION

SELECT distinct colA, colB
FROM car
JOIN (SELECT COUNT(*) c FROM make) make
JOIN model ON car.id = model.id
WHERE make.c = 0

将LEFT JOIN版本扩展到两个表格非常简单：

SELECT DISTINCT colA, colB
FROM car
JOIN (SELECT COUNT(*) c FROM make) makecount
LEFT JOIN make ON car.id = make.id
JOIN (SELECT COUNT(*) c FROM model) modelcount
LEFT JOIN model ON car.id = model.id
WHERE (makecount.c = 0 OR make.id IS NOT NULL)
  AND (modelcount.c = 0 OR model.id IS NOT NULL)

如果要加入其他表格，您可以继续重复此模式。

使用UNION查询执行此操作更难，因为对于每个可以为空的连接表组合，您需要一个子查询：具有行的make和model的一个子查询，仅用于make，一个仅用于model，另一个用于空。如果有3个表连接，则会扩展到8个子查询（即总有2个ⁿ子查询）。也许有人可以想出一个更好的方法，我无法想到这一点。

Answer 2

您必须使用更复杂的联接来执行此操作：

select c.colA, c.colB
from car c cross join
     (select count(*) as num from make) m
where num = 0 or
      (c.id in (select id from make) and 
       c.id in (select id from model)
      )

Answer 3

试试这个

     select colA, colB from car
     left join make on make.id = car.id
     inner join model on model.id = car.is
     where make.id IS NOT NULL

sql中的条件Where子句

3 个答案: