Selecting data only if related data exists

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Hi,

I have the following query to return a bulk 'exists' query on multiple records. What I'm looking to do is only to select u.role and several other fields if the username exists in 'users'. This method works, but it's messy as I'm checking if the username exists for every column I'm selecting. Is there a more compact way to say;

If the username exists, return the role, x, y, z column etc.
Otherwise, return NULL for the role, x, y, z column etc.

CREATE TEMPORARY TABLE _users(username varchar(256));
        INSERT INTO _users VALUES('testuser1', 'testuser2');
        SELECT _u.username,
        CASE
                WHEN ISNULL(u.username) = 1 THEN 0
                ELSE 1
        END 'exists',
        CASE
                WHEN ISNULL(u.username) = 1 THEN u.role
                ELSE NULL
        END 'role'
        FROM _users _u
        LEFT JOIN users u
        ON u.username = _u.username;
        DROP TABLE _users

What I have tried:

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Posted 20-Mar-18 23:52pm

User 13204940

Updated 20-Mar-18 23:56pm

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[no name] 21-Mar-18 17:50pm

I’ve already said that the query works perfectly. I’m using MySQL, which only has one parameter for ISNULL.

Richard Deeming 21-Mar-18 17:54pm

Where in your question did you mention MySQL?

You could at least have used the "MySQL" tag, rather than the generic "SQL" tag. MySQL has so many differences from regular SQL that it might as well be a different language.

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Maciej Los · Answer 1 · 2018-03-20T23:58:00

I'd recommend to read this: EXISTS (Transact-SQL)[^]

[EDIT]
If you would like to return NULL, you can use this trick:

SQL

SELECT TOP(1) Field1, Field2, Field3 FROM YourTable WHERE UserName = 'Whatever' UNION ALL SELECT NULL AS Field1, NULL AS Field2, NULL As Field3

But - i do NOT recommend to do such a thing for several reason...

Here's a 'working' example:

SQL

DECLARE @t TABLE (uName VARCHAR(30)) INSERT INTO @t (uName) VALUES('A'), ('B'), ('C') SELECT TOP(1) uName FROM @t WHERE uName = 'D' UNION ALL SELECT NULL AS uName --returns: NULL

[EDIT 2]
According to our discussion, a LEFT JOIN is doing exactly what you do expect to achieve. See:

DECLARE @tmp TABLE (uName VARCHAR(30))
INSERT INTO @tmp (uName)
VALUES('A'), ('B'), ('C')


DECLARE @orig TABLE (uName VARCHAR(30), [role] VARCHAR(30))
INSERT INTO @orig (uName, [role])
VALUES('A', 'admin'), ('D', 'user'), ('C', 'user')

SELECT t.uName AS TmpName, o.*
FROM @tmp t LEFT JOIN @orig o ON t.uName = o.uName

Result:

TmpName	uName	role
A		A		admin
B		NULL	NULL
C		C		user

For further details, please see: Visual Representation of SQL Joins[^]