找出一个SQL表是否有条目没有“标记”记录

Question

我有一个MySQL表的地址有以下的列：

STATE, ZIPCODE, FLAG 

可以有一个给定state/zipcode组合多行。我想确保对于每个给定的state/zipcode组合，只有一行有FLAG = 'P'。

我知道如何检查多行：

SELECT state, zipcode, flag, count(*) as numrecs 
FROM addresses 
WHERE flag = 'P' 
group by state, zipcode 
having numrecs > 1; 

如何检查零个条目 - 一个state/zipcode组合没有行其flag = 'P'？

Answer 1

该查询应该给你违反你的准则的：？

SELECT STATE, ZIPCODE, SUM(FLAG_COUNTER) 
FROM (SELECT *, IF(FLAG='P', 1, 0) AS FLAG_COUNTER FROM addresses) tableWithCounter 
GROUP BY STATE, ZIPCODE 
HAVING SUM(FLAG_COUNTER) != 1 

Answer 2

你是在正确的轨道上，只是使用条件汇总：

SELECT state, zipcode, flag, count(*) as numrecs 
FROM addresses 
GROUP BY state, zipcode 
HAVING coalesce(sum(flag = 'P'), 0) <> 1 

或者，如果你喜欢：

HAVING coalesce(sum(flag = 'P'), 0) = 0 or sum(flag = 'P') > 1 

的coalesce()需要，以防万一flag可以NULL和给定州/邮编组合只有NULL标志。