SQL 服务器 select 语句的问题

Question

I need help with a select statement that I'm currently struggling with. The scenario is this:

select * from mytable WHERE isActive=1

1. and if only id is provided, select everything where the id=@id
2. if firstname and lastname is provided, then select everything where firstname=@firstname and lastname=@lastname regardless of the ID
3. if either firstname or lastname is provided, select everything where firstname=@firstname or lastname=@lastname regardless of the ID again
4. if id, firstname and lastname provided, just select where firstname=@firstname and lastname=@lastname regardless of the ID again

Here's my query:

DECLARE @ID INT =25,
@firstname NVARCHAR(100),
@lastname NVARCHAR(100);
SELECT * from mytable
where isActive=1
and ( ID = -1 or ID = @ID) 
or (firstname = @firstname)
or (lastname = @lastname) 
or (firstname = @firstname and lastname = @lastname)

somehow I'm not getting the results expected, for example when I provide both firstname and lastname, it shows everyone with the firstname regardless of their lastname:-(

Please help

Answer 1

Not sure if you need a union, especially UNION ALL, but you have two choices. First one is pretty, it works with small data sets. The second option looks ugly, but gives you almost guaranteed best performance.

SELECT * FROM mytable WHERE isActive=1
    AND (
        (@ID IS NOT NULL AND @firstname IS NULL AND @lastname IS NULL AND @ID = ID) OR
        (@firstname IS NOT NULL AND @lastname IS NOT NULL AND firstname=@firstname and lastname=@lastname) OR
        (@firstname IS NOT NULL AND @lastname IS NULL AND firstname=@firstname) OR
        (@firstname IS NULL AND @lastname IS NOT NULL AND lastname=@lastname)
    )

---

IF @ID IS NULL AND @firstname IS NULL AND @lastname IS NULL
SELECT * FROM mytable WHERE isActive=1;

IF @ID IS NOT NULL AND @firstname IS NULL AND @lastname IS NULL
SELECT * FROM mytable WHERE isActive=1 AND @ID = ID;

IF @firstname IS NOT NULL AND @lastname IS NOT NULL 
SELECT * FROM mytable WHERE isActive=1 AND firstname=@firstname AND lastname=@lastname;

IF @firstname IS NOT NULL AND @lastname IS NULL 
SELECT * FROM mytable WHERE isActive=1 AND firstname=@firstname;

IF @firstname IS NULL AND @lastname IS NOT NULL 
SELECT * FROM mytable WHERE isActive=1 AND lastname=@lastname;

That is with no difference between #2 & #4, unless you can describe it.

Answer 2

Please try the following method. Though the WHERE clause predicates are not SARGable.

SQL

DECLARE @ID INT =25
    , @firstname NVARCHAR(100) = NULL
    , @lastname NVARCHAR(100) = NULL;

SELECT * 
FROM mytable
WHERE isActive = 1
    AND ID = COALESCE(@ID, -1) -- or COALESCE(@ID, ID) depending on the logic
    AND firstname = COALESCE(@firstname, firstname)
    AND lastname = COALESCE(@lastname, lastname);

Answer 3

It is possible to write these as one query, but owing to the way indexes are used, it will probably result in an inefficient table scan. Therefore you are best off using UNION ALL to join the separate queries:

;WITH cte AS (
    select * from mytable WHERE isActive=1
)
SELECT *
    FROM cte
    WHERE firstname = @firstname AND @lastname IS NULL
UNION ALL
SELECT *
    FROM cte
    WHERE lastname = @lastname AND @firstname IS NULL
UNION ALL
SELECT *
    FROM cte
    WHERE firstname = @firstname AND lastname = @lastname
UNION ALL
SELECT *
    FROM cte
    WHERE ID = @ID
        AND @firstname IS NULL AND AND @lastname IS NULL
OPTION(OPTIMIZE FOR UNKNOWN);

The UNKNOWN bit hints to the compiler that you don't know what may be entered, and it shouldn't keep the parameter values from the first run (parameter sniffing).

An alternative hint is OPTION(RECOMPILE) , which means the query will be recompiled on every run. This has overhead, though.

Answer 4

Write it as a series of IF statements:

--Test #1
DECLARE @ID INT = 25, @firstname NVARCHAR(100) = 'Florian', @lastname NVARCHAR(100) = 'Voss';

IF (@firstname IS NOT NULL AND @lastname IS NOT NULL)
    SELECT *
    FROM [AdventureWorks2019].[Person].[Person]
    WHERE (firstname = @firstname AND lastname = @lastname)

IF @firstname IS NOT NULL AND @lastname IS NULL
    SELECT *
    FROM [AdventureWorks2019].[Person].[Person]
    WHERE (firstname = @firstname)
GO

--Test #2
DECLARE @ID INT = 25, @firstname NVARCHAR(100) = 'Florian', @lastname NVARCHAR(100) = NULL;

IF (@firstname IS NOT NULL AND @lastname IS NOT NULL)
    SELECT *
    FROM [AdventureWorks2019].[Person].[Person]
    WHERE (firstname = @firstname AND lastname = @lastname)

IF (@firstname IS NOT NULL AND @lastname IS NULL)
    SELECT *
    FROM [AdventureWorks2019].[Person].[Person]
    WHERE (firstname = @firstname)
GO