MySQL如何根据不同条件联查不同表的数据if/case

IF(expr1,expr2,expr3)

SELECT product_id, 
IF(product_type_id=1, (SELECT COUNT(*) FROM `device` WHERE product_id = product.product_id),
					  (SELECT COUNT(*) FROM `monitor`WHERE product_id = product.product_id)) as device_num
FROM `product`
WHERE product_id in (1,25)

CASE expr1 WHEN a THEN A WHEN … THEN … ELSE … END

SELECT product_id,
CASE product_type_id 
WHEN 1 THEN (SELECT COUNT(*) FROM `device` WHERE product_id = product.product_id)
WHEN 2 THEN (SELECT COUNT(*) FROM `monitor`WHERE product_id = product.product_id)
	   ELSE (SELECT COUNT(*) FROM `other_device`WHERE product_id = product.product_id) END as device_num
FROM `product`
WHERE product_id in (1,11,25)

select DISTINCT   
ifnull((select sum(‘列名') from a, b where a.id = b.id and a.condition=condition1 and b.condition = condition2),0) as '条件1下的数据' ,
ifnull((select sum(‘列名') from a, b where a.id = b.id and a.condition=condition3 and b.condition = condition4),0) as '条件2下的数据'
from a, b where a.id = b.id 

select ‘别名1', ‘别名2'
from 
(select IFNULL(sum(‘列名'),0) ‘别名1' from a, b where a.id = b.id and a.condition=condition1 and b.condition = condition2),0) t1
left join 
(select IFNULL(sum(‘列名'),0) ‘别名2' from a, b where a.id = b.id and a.condition=condition3 and b.condition = condition4) t2 on 1=1