请说明原因,1.SELECTA.FIRST_NAME FROM EMPLOYEES A LEFT JOIN EMPLOYEES B ON A.EMPLOYEE_ID=B.MANAGER_ID WHERE B.FIRST_NAME ='David' 2.SELECT A.FIRST_NAME FROM EMPLOYEES A ,EMPLOYEES B WHERE A.EMPLOYEE_ID=B.MANAGER_ID AND B.FIRST_NAME ='David'

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请说明原因,
1.SELECTA.FIRST_NAME FROM EMPLOYEES A LEFT JOIN EMPLOYEES B ON A.EMPLOYEE_ID=B.MANAGER_ID WHERE B.FIRST_NAME ='David'
2.SELECT A.FIRST_NAME FROM EMPLOYEES A ,EMPLOYEES B WHERE A.EMPLOYEE_ID=B.MANAGER_ID AND B.FIRST_NAME ='David'

下面的,下面的属于内联,外联要比内联多一步添加主表的非关联性数据