sql - 使用join仅检索第一条记录

Question

我有三个表：Employee，Contact，Employee_Contact

员工：

EMP_ID, FIRST_NAME, LAST_NAME...

联系方式：

CONTACT_ID, TELEPHONE, MOBILE...

EMPLOYEE_CONTACT：

EMP_ID, CONTACT_ID

Employee和Contact表与Employee_Contact表映射。 所以我想用Employee唯一的第一次联系来检索所有员工，我尝试了这个查询，但它正在检索与该员工相关的所有联系人。请检查以下查询，如有任何错误，请通知我。

SELECT EMP.FirstName
    ,DEPT.Description AS Department
    ,CNT.Mobile
    ,CNT.Telephone
FROM MstEmp EMP
LEFT OUTER JOIN Department DEPT ON EMP.DeptID = DEPT.Department_ID
LEFT OUTER JOIN Employee_Contact EC ON EMP.EMP_ID = EC.EMP_ID
OUTER APPLY (
    SELECT TOP 1 *
    FROM Contact CONT
    WHERE EC.Contact_ID = CONT.Contact_ID
    ) CNT

Answer 1

使用Top关键字检索前一条记录。

SELECT TOP 1 EMP.FirstName, DEPT.Description AS Department, CNT.Mobile, 
CNT.Telephone
FROM MstEmp EMP
LEFT OUTER JOIN Department DEPT on EMP.DeptID = DEPT.Department_ID
LEFT OUTER JOIN Employee_Contact EC ON EMP.EMP_ID = EC.EMP_ID
OUTER APPLY(SELECT TOP 1 * FROM Contact CONT WHERE EC.Contact_ID = 
CONT.Contact_ID)CNT

Answer 2

试试这个：

SELECT EMP.FirstName
    ,DEPT.Description AS Department
    ,t.Mobile
    ,t.Telephone
FROM MstEmp EMP
LEFT OUTER JOIN Department DEPT ON EMP.DeptID = DEPT.Department_ID
LEFT OUTER JOIN Employee_Contact EC ON EMP.EMP_ID = EC.EMP_ID
LEFT OUTER JOIN Contact CONT ON EC.Contact_ID = (
        SELECT MIN(c.Contact_ID) c_id
        FROM Contact c
        WHERE CONT.Contact_ID = c.Contact_ID
        ) t

Answer 3

您可以使用ROW_NUMBER()获取所有员工的第一条记录，然后将其与其他表格一起加入。

SELECT EMP.FirstName,
       DEPT.Description AS Department,
       c.Mobile,
       c.Telephone
FROM MstEmp EMP
LEFT OUTER JOIN Department DEPT ON EMP.DeptID = DEPT.Department_ID
INNER JOIN (
             SELECT *,ROW_NUMBER() OVER (PARTITION BY EMP_ID ORDER BY EMP_ID) AS RN
             FROM Employee_Contact  ) EC ON EMP.EMP_ID = EC.EMP_ID
INNER JOIN Contact c ON c.Contact_ID=EC.Contact_ID
WHERE EC.RN=1

Answer 4

试一试：

select * from (
select E.[emp_id],
       E.[first_name], 
       E.[last_name], 
       C.*, 
       RANK() over (partition by (E.[emp_id], E.[first_name], E.[last_name]) order by (select null)) [RN]
from Employee [E]
join employee_contact [EC] on E.[emp_id] = EC.[emp_id]
join contact [C] on EC.[contact_id] = C.[contact_id]
) A where RN = 1