sql_isnull对查询结果无返回值的处理

如果查询结果无返回值（查询不到数据），可用 case when + count() 进行处理

select isnull((SELECT a.status FROM t_a a WHERE a.id = tc.id AND a.flag = 1),0)

SELECT a.status FROM t_a a WHERE a.id = tc.id AND a.flag = 1
上面这条sql 可能无数据返回，导致执行isnull，数据库会进行类型转换，可能长时间一直在查询中

优化处理：

select case when
(select count(status) FROM t_a a WHERE a.id = tc.id AND a.flag = 1) = 1
then (SELECT a.status FROM t_a a WHERE a.id = tc.id AND a.flag = 1 )
else 0 
end
)

https://blog.csdn.net/icecoola_/article/details/79151488