两个字段连接问题产生的思考

-- ① 运行报错
SELECT  a. Employee_id,  DISTINCT b. Employee_id
FROM Employee a 
LEFT JOIN Incentive b ON a. Employee_id = b. Employee_id

-- ② 运行正常
SELECT DISTINCT a. Employee_id,  b. Employee_id
FROM Employee a 
LEFT JOIN Incentive b ON a. Employee_id = b. Employee_id

DISTINCT 的位置不同导致的差异！ ？②为啥会报错！暂且记住第二种方法