sql优化问题啊，怎么去掉UNION啊

Question

帮我把这段SQL优化一下啊，去掉UNION ALL

SELECT DISTINCT
CL.NAME
FROM
COURSES PC
JOIN MAJORGRADE MG ON MG.ID = PC.ID
JOIN CLASS CL ON CL.NJ = MG.NJ AND CL.ZYBH = MG.ZYBH AND CL.LDM IS NULL

union all

SELECT DISTINCT
CL.NAME
FROM
COURSES PC
JOIN MAJORGRADE MG ON MG.ID = PC.ID
JOIN CLASS CL ON CL.NJ = MG.NJ AND CL.LDM = MG.LDM

Answer 1

可以改用成代码获取数据，分成多个子查询，最后Java代码合并数据