在同一 table 上两次获取左连接的空数据

Question

我有一些这样的表：

员工：

EmployeeId      Name        DeptId
----------------------------------
100             John        100
101             Kery        100
102             Tiny        100

团队：

TeamId      TeamLeaderId    TeamName
------------------------------------------
1           100             Finance-2021

团队成员：

TeamMemberId    TeamId      EmployeeId
----------------------------------------
1               1           101

我正在尝试获取员工名单以及有关他们是谁的信息：Team Member or Team Leader or Individual。

当前输出：

TeamId      TeamName        EmployeeIdentity    EmployeeId    EmployeeName
------------------------------------------------------------------
null        null            Team Leader         100             John
1           Finance-2021    Team Member         101             Kery

预期输出：

TeamId      TeamName        EmployeeIdentity    EmployeeId      EmployeeName
-----------------------------------------------------------------------
1           Finance-2021    Team Leader         100             John
1           Finance-2021    Team Member         101             Kery

查询：

INSERT INTO #tempMembers (ID)
    (SELECT EmployeeId 
     FROM TeamMembers
     WHERE TeamId IN (1) AND IsDeleted = 0
     UNION   
     SELECT TeamLeaderId 
     FROM TEAMS
     WHERE TeamId IN (1))

SELECT
    t.TeamId as 'TeamId',
    t.TeamName as 'TeamName',
    CASE 
        WHEN tm.EmployeeId IS NOT NULL THEN 'Team Member'
        WHEN t1.TeamLeaderId IS NOT NULL THEN 'Team Leader'
        ELSE 'Individual'
    END AS 'EmployeeIdentity',
    e.EmployeeId as 'EmployeeId',
    e.EmployeeName as 'EmployeeName',
FROM
    Employee AS e
LEFT JOIN
    TeamMember tm ON tm.EmployeeId = e.EmployeeId AND tm.IsDeleted = 0
LEFT JOIN
    Teams t ON t.TeamId = tm.TeamId
LEFT JOIN
    Teams t1 ON t1.TeamLeaderId = e.EmployeeId
WHERE
    e.IsDeleted = 0 
    AND e.EmployeeId IN (SELECT * FROM #tempMembers)

这里唯一的问题是我得到的“团队负责人”数据 TeamId and TeamName 为空白，如下所示：

当前输出：

TeamId      TeamName        EmployeeIdentity    EmployeeId      EmployeeName
null        null            Team Leader         100             John

谁能帮我解决这个问题？

Answer 1

这是一种方法：

select 
    t.teamid, t.teamName, 'Team Member' as EmployeeIdentity  ,    
    e.employeeid , e.emplyeename 
from
    Employee e 
join 
    TeamMember tm on tm.employeeid = e.employeeid 
join 
    teams t on tm.TeamId = t.TeamId
union all 
select 
    t.teamid, t.teamName, 'Team Leader' as EmployeeIdentity, 
    e.employeeid , e.emplyeename  
from 
    employees e 
join 
    Teams t on t.teamleaderid = e.employeeid

甚至更好：

select 
    t.teamid, t.teamName, EmployeeIdentity, e.employeeid, e.emplyeename 
from 
    Employee e 
join
    (select 
         TeamId, Employeeid, 'Team Member' as EmployeeIdentity 
     from TeamMember 
     union all 
     select teamid, TeamLeaderId, 'Team Leader' as EmployeeIdentity 
     from team) as tm on tm.employeeid = e.employeeid 
join 
    teams t on tm.TeamId = t.TeamId

Answer 2

我认为您的问题是您从 t 获取 teamid 和 teamname，而在领导案例中它来自 t1。试试这个：

Case When t.TeamId is null then t1.TeamId else t.TeamId end as 'TeamId',
Case When t.TeamName is null then t1.TeamName else t.TeamId end as 'TeamName',

Answer 3

此方法不需要临时 table 并使用左联接中的空值来确定成员是 Team Member、Team Leader 还是 Individual。

where 条件应用来自临时联合的过滤器 table。

我还在下面包含了一个 db-fiddle 来演示。

SELECT
    CASE
       WHEN tl.TeamLeaderId IS NOT NULL THEN tl.TeamId
       WHEN tmt.TeamId IS NOT NULL THEN tmt.TeamId
       ELSE NULL
    END as TeamId,
    CASE
       WHEN tl.TeamLeaderId IS NOT NULL THEN tl.TeamName
       WHEN tmt.TeamName IS NOT NULL THEN tmt.TeamName
       ELSE NULL
    END as TeamName,
    CASE
        WHEN tl.TeamLeaderId IS NOT NULL THEN 'Team Leader'
        WHEN tm.EmployeeId IS NOT NULL THEN 'Team Member'
        ELSE 'Individual'
    END as EmployeeIdentity,
    e.EmployeeId,
    e.Name as EmployeeName
FROM
    Employee e
LEFT JOIN
    Teams tl ON e.EmployeeId = tl.TeamLeaderId AND tl.isDeleted=0
LEFT JOIN
    TeamMember tm ON e.EmployeeId = tm.EmployeeId AND tm.isDeleted=0
LEFT JOIN
    Teams tmt ON tm.TeamId = tmt.TeamId AND tmt.isDeleted=0
WHERE 
    tl.TeamId = 1 OR tmt.TeamId=1

TeamId	TeamName	EmployeeIdentity	EmployeeId	EmployeeName
1	Finance-2021	Team Leader	100	John
1	Finance-2021	Team Member	101	Kery

View on DB Fiddle or SQL Server Db Fiddle

让我知道这是否适合你。

Answer 4

您可以 select 团队领导和团队成员加入 APPLY，然后加入 Employee。不需要自连接或临时表

SELECT
  t.TeamId,
  t.TeamName,
  m.EmployeeIdentity,
  e.EmployeeId,
  e.Name AS EmployeeName
FROM Teams t
CROSS APPLY (
    SELECT
      t.TeamLeaderId AS EmployeeId,
      'Team Leader' AS EmployeeIdentity
    UNION ALL
    SELECT
      tm.EmployeeId,
      'Team Member'
    FROM TeamMember tm
    WHERE tm.TeamId = t.TeamId
) m
JOIN Employee e ON e.EmployeeId = m.EmployeeId
  AND e.IsDeleted = 0
WHERE t.TeamId = 1;

db<>fiddle.uk

在同一 table 上两次获取左连接的空数据

Getting null data for left join on same table twice

sql

sql-server

left-join