多表无条件从子查询混淆中获取行

Question

INSERT ALL
 INTO sal_history (empid,hiredate,sal)
 VALUES (empid,hiredate,sal)
 INTO mgr_history (empid,mgr,sal)
 VALUES (empid,mgr,sal)
SELECT employee_id AS "EMPID",
       hire_date   AS "HIREDATE",
       salary      AS "SAL",
       manager_id  AS "MGR"
FROM   employees
WHERE  employee_id > 200;

insert into子句如何在整个语句中知道在多表sal_history和mgr_history中选择哪些列进行插入？我可以看到sal_history中有3列，mgr_history中有3列，但在子查询中有4列。

我知道这个语句是如何工作的，对于子查询中的每一行，＆＃34; insert_into_clause＆＃34;只执行一次，但这不是重点，只是这个小小的东西，我不了解列。

Answer 1

每个INTO子句指定要为该表填充的列，匹配的VALUES子句指定要使用的子查询中的哪些列。在这种情况下，子查询使用相同的名称，这可能有点令人困惑;也许你会发现不同的别名更清楚：

INSERT ALL
 INTO sal_history (empid,hiredate,sal)
 VALUES (sub_empid,sub_hiredate,sub_sal)
 INTO mgr_history (empid,mgr,sal)
 VALUES (sub_empid,sub_mgr,sub_sal)
SELECT employee_id AS "SUB_EMPID",
       hire_date   AS "SUB_HIREDATE",
       salary      AS "SUB_SAL",
       manager_id  AS "SUB_MGR"
FROM   employees
WHERE  employee_id > 200;

VALUES子句中使用了所有四个子查询列（别名）;每个都有三个，SUB_EMPID和SUB_SAL（带有希望澄清的前缀）出现在两者中。