Question

SQL效率问题，想问下下面三条语句，哪条效率更高一点？

SELECT ISNULL(T1.TC001,T2.TC001) TC001,ISNULL(T1.TC002,T2.TC002) TC002
FROM puri..COPTC T1
FULL OUTER JOIN puribk..COPTC T2 ON T1.TC001=T2.TC001 AND T1.TC002=T2.TC002

SELECT TC001,TC002
FROM puri..COPTC T1
UNION ALL
SELECT TC001,TC002
FROM puribk..COPTC T2
WHERE NOT EXISTS(SELECT 1 FROM puri..COPTC T1 WHERE T1.TC001=T2.TC001 AND T1.TC002=T2.TC002)

SELECT TC001,TC002
FROM puri..COPTC T1
UNION ALL
SELECT TC001,TC002
FROM puribk..COPTC T2
WHERE ISNULL((SELECT COUNT(*) FROM puri..COPTC T1 WHERE T1.TC001=T2.TC001 AND T1.TC002=T2.TC002),0)>0

Answer 1

第一条记录理论上性能最好。

追问

哦，可是从语句的书写效率来说却是最麻烦的。
那下面那两条，那条效率快点呢？