SQL联接左联接或左外部联接

Question

我在SQL Join中有一个问题。 我有以employeeid作为主键的表employee，以及employee的其他一些列。 还有另一个表称为employeeaddress，其中可以有多个employeeid是一个外键。 一位员工可以拥有许多员工地址，仅用于解释一对多关系。

如果我想编写一个查询，它将获取以下列

 employee.employeeid, employee.empname,
 employeeaddress.employeeaddressid, employeeaddress.addr1,
 employeeaddress.addr2

因此，可能有一个没有雇员地址的雇员。 但是无论如何，我想获取所有可能有零个或多个地址的员工。

我需要应用left join还是left outer join ？ 我想要具有2个雇员John和Michael的表的以下结果，其中John具有两个employeeaddresses ，其employeeaddressid 21和22而Michael没有employeeaddress

1, John, 21, addr1 for John, addr2 for John
1, John, 22, another addr1 for John, another addr2 for John
2, Michael, NULL , NULL , NULL

以上结果按以下方式排列

employee.employeeid, employee.empname, employeeaddress.employeeaddressid, employeeaddress.addr1, employeeaddress.addr2

请帮忙。

Answer 1

根据您的描述，听起来您正在按以下方式查找查询。 如果还需要地址详细信息，则只需向外部查询添加左连接。

同样，正如所躲避的评论一样， LEFT JOIN是LEFT OUTER JOIN简写，它们将产生相同的结果。

SELECT *
FROM employee
inner join
(
    SELECT
        employeeid,
        count(*) as addresscount
    FROM employee
    left join employeeaddress ON employeeaddress.employeeaddressid = employee.employeeaddressid
    group by employeeid
) counts on counts.employeeid = employee.employeeid
WHERE counts.addresscount = 0 -- Or 1, or 5 or > 1, etc.

Answer 2

只需LEFT JOIN 。

SQL小提琴示例

SELECT  e.employeeID ,
        e.empName ,
        ea.employeeAddressID ,
        ea.addr1 ,
        ea.addr2
FROM    Employee e
        LEFT JOIN EmployeeAddress ea ON ea.employeeID = e.employeeID