How to set an if on a select for returning select in sql?

问题

I have two tables named User and ParentUser that there is an one to many relation between them and the many side is the ParentUser table.

I have written this select on User table:

select u.* from [User] u inner join ParentUser p on u.UserId=p.UserId where p.ParentId=2440

Now I wanna add another column to u.* containing 0 or 1.0 is for users who have some children in ParentUser and 1 is for those whom dont have any.

How to handle this?

Update

回答1:

You need LEFT JOIN

SELECT u.*,
       CASE
         WHEN p.ParentId IS NULL THEN 1
         ELSE 0
       END AS HasChildren
FROM   [User] u
       LEFT JOIN (SELECT DISTINCT ParentId
                  FROM   ParentUser
                  WHERE  ParentId IN (SELECT UserId
                                      FROM   ParentUser
                                      WHERE  parentId = 2440)) p
              ON u.UserId = p.ParentId

回答2:

SELECT DISTINCT
     [UserId]       =   [u].[userid]
    ,[HasChildren]  =   CAST(ISNULL([pu].[userid], 0) AS BIT)
FROM
    [User]  AS  [u]
OUTER APPLY
    (
        SELECT [userid] FROM [ParentUser] WHERE [parentid] = [u].[userid]
    )       AS  [pu];

And, if you want to filter by parent id:

 DECLARE @ParentId INT = 2;

SELECT DISTINCT
     [UserId]       =   [u].[userid]
    ,[HasChildren]  =   CAST(ISNULL([pu].[userid], 0) AS BIT)
    ,[ChildrenId]   =   [pu].[userid]
FROM
    [user]  AS  [u]
OUTER APPLY
    (
        SELECT [userid], [parentid] FROM [ParentUser] WHERE [parentid] = [u].[userid]
    )       AS  [pu]
WHERE
        [pu].[parentid] = @ParentId;

回答3:

It's fairly easy to determine each user that has children. The following starts out by determining the number of children, but in final output (with other columns from user) simply reduces this to: "Are there any children?"

/************************* Set up sample data *************************/
declare @User table (
    UserId int,
    UserName varchar(10)
)
insert @User
select 1, 'A' union all
select 2, 'B' union all
select 3, 'C'

declare @ParentUser table (
    UserId int,
    ParentId int
)
insert @ParentUser
select 3, 1 union all
select 3, 2 union all
select 2, 1


/****************************** Solution ******************************/
;with ChildCounts as (
    select  p.ParentId, COUNT(*) as ChildCount
    from    @ParentUser p
    group by p.ParentId
), UsersWithHasChildren as (
    select  u.*,
            case
                when p.ChildCount > 0 then 1
                else 0
            end as HasChildren
    from    @User u
            left join ChildCounts p on
                p.ParentId = u.UserId
)
select  *
from    UsersWithHasChildren u

From your duplicate question, the special case of determining which children of a particular parent also have children is a trivial WHERE filter as below.

where   u.UserId in (
            select  p.UserId
            from    @ParentUser p
            where   p.ParentId = 1 /*Parametrise this value*/
        )

来源：https://stackoverflow.com/questions/41669968/how-to-set-an-if-on-a-select-for-returning-select-in-sql

标签

sql

sql-server

sqlcommand