从执行计划上看，是exist的耗费低；但是执行的时间，是表关联的用时少。
sql如下：SELECT COUNT(1) -- 耗费 9895 ，用时1.172s/1.532s/1.231s
FROM table1 A, table2 B
WHERE A.JOB_CODE IS NOT NULL AND B.FACTDETAILNO = '5005'
AND (B.FACTDETAILCODE = SUBSTR(A.JOB_CODE, 1, 3) OR
B.FACTDETAILCODE = SUBSTR(A.JOB_CODE, 1, 2));
SELECT COUNT(1)-- 耗费 5950 ，用时3.721s/2.781s/4.562s
FROM table1 A
WHERE A.JOB_CODE IS NOT NULL
AND EXISTS (SELECT 1
FROM table2 B AND B.FACTDETAILNO = '5005'
WHERE (B.FACTDETAILCODE = SUBSTR(A.JOB_CODE, 1, 2) OR
B.FACTDETAILCODE = SUBSTR(A.JOB_CODE, 1, 3))
)
表关联，与exist的查询过程是怎么查询的，哪个更优些，或者分情况。
