找出一個SQL表是否有條目沒有「標記」記錄

Question

我有一個MySQL表的地址有以下的列：

STATE, ZIPCODE, FLAG 

可以有一個給定state/zipcode組合多行。我想確保對於每個給定的state/zipcode組合，只有一行有FLAG = 'P'。

我知道如何檢查多行：

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

如何檢查零個條目 - 一個state/zipcode組合沒有行其flag = 'P'？

Answer 1

該查詢應該給你違反你的準則的：？

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

你是在正確的軌道上，只是使用條件彙總：

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

或者，如果你喜歡：

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

的coalesce()需要，以防萬一flag可以NULL和給定州/郵編組合只有NULL標誌。