Table-Valued Function using IF statement in SQL Server - sql

Question

I have a Student table consists of following parameters
[ID] [nvarchar](50) NOT NULL,
[Firsname] [nvarchar](50) NOT NULL,
[Lastname] [nvarchar](50) NOT NULL,
[Melicode] [nchar](10) NOT NULL,
[City] [nvarchar](50) NOT NULL,
[Province] [nvarchar](50) NOT NULL,
[Active] [int] NULL
i want to write a Table-Valued Function named Show which has one parameter as number. the function will act as following
if #number = 1 , returns all columns from Student table
if #number = 2 , returns only City from Student
if #number = 3 , returns only Province from Student
i wrote the following T-SQL, but it only works for (if (#number = 1)). When the user enter #number as 2 or 3, the function does not work. Thank You
Create function Show(#number int)
RETURNS #result TABLE
(
[ID] [nvarchar](50) NOT NULL,
[Firsname] [nvarchar](50) NOT NULL,
[Lastname] [nvarchar](50) NOT NULL,
[Melicode] [nchar](10) NOT NULL,
[City] [nvarchar](50) NOT NULL,
[Province] [nvarchar](50) NOT NULL,
[Active] [int] NULL
)
AS
BEGIN
IF (#number = 1)
INSERT INTO #result SELECT * from Student
IF (#number = 2)
INSERT INTO #result (City) values ((SELECT City from Student))
IF (#number = 3)
INSERT INTO #result (Province) values ((SELECT Province from Student))
RETURN -- #Players (variable only required for Scalar functions)
END
go
select *from dbo.show(1)

Answer 1

This is not going to work:
INSERT INTO #result (City)
VALUES ((SELECT City from Student))
Either you have all the values as scalar SQL variables, or literals - then you can use
INSERT INTO #result (City)
VALUES ('New York')
INSERT INTO #result (City)
VALUES (#ChosenCity)
or you have a SELECT statement to fill the values - then you need this syntax:
INSERT INTO #result (City)
SELECT City
FROM Student
without the VALUES keyword. And as #GiorgiNakeuri correctly states - this will then fail because all your columns require a value (have the NOT NULL attribute), so this insert cannot succeed - you need to provide all NOT NULL values (or define a default value for each column)

Answer 2

CREATE FUNCTION dbo.Show
(
#number INT
)
RETURNS #result TABLE
(
ID NVARCHAR(50),
Firsname NVARCHAR(50),
Lastname NVARCHAR(50),
Melicode NCHAR(10),
City NVARCHAR(50),
Province NVARCHAR(50),
Active INT
)
AS
BEGIN
IF (#number = 1)
INSERT INTO #result
SELECT * FROM dbo.Student
IF (#number = 2)
INSERT INTO #result (City)
SELECT City FROM dbo.Student
IF (#number = 3)
INSERT INTO #result (Province)
SELECT Province FROM dbo.Student
RETURN
END
GO
SELECT * FROM dbo.Show(2)

Answer 3

the table returned is dictated by how the result table was declared. the query below works (in a sense) but the results include all the columns with NULLs for those columns not targeted by the #number parameter:
CREATE TABLE dbo.z_Show (str1 VARCHAR(10), str2 VARCHAR(10), str3 VARCHAR(10))
INSERT z_show
SELECT 1, 1, 1 UNION ALL
SELECT 2, 2, 2 UNION ALL
SELECT 3, 3, 3
CREATE FUNCTION dbo.Show(#number int)
RETURNS #result TABLE
(
--[ID] [nvarchar](50) NOT NULL,
--[Firsname] [nvarchar](50) NOT NULL,
--[Lastname] [nvarchar](50) NOT NULL,
--[Melicode] [nchar](10) NOT NULL,
--[City] [nvarchar](50) NOT NULL,
--[Province] [nvarchar](50) NOT NULL,
--[Active] [int] NULL
str1 VARCHAR(10), str2 VARCHAR(10), str3 VARCHAR(10)
)
AS
BEGIN
--for debugging|start
--DECLARE #number INT = 3
--DECLARE #result TABLE (str1 VARCHAR(10), str2 VARCHAR(10), str3 VARCHAR(10))
--for debugging|end
IF (#number = 1)
BEGIN
--PRINT ('IF (#number = 1)')
INSERT INTO #result SELECT * from dbo.z_Show
END
IF (#number = 2)
BEGIN
--PRINT ('IF (#number = 2)')
INSERT INTO #result (str2) SELECT str2 from dbo.z_Show
END
IF (#number = 3)
BEGIN
--PRINT ('IF (#number = 3)')
INSERT INTO #result (str3) SELECT str3 from dbo.z_Show
END
RETURN -- #Players (variable only required for Scalar functions)
END
SELECT 'number 1 was passed', *
FROM dbo.show(1)
SELECT 'number 2 was passed', *
FROM dbo.show(2)
SELECT 'number 3 was passed', *
FROM dbo.show(3)

Answer 4

You mentioned #result has all NOT NULL columns. If you want to insert only city into that #result, it will take remaining columns as Null so that's why an error happened. You don't mention that #result columns are NOT NULL columns and one more is. Remove VALUES keyword from the INSERT statement because it is inserting with a Select statement

Answer 5

The insert statements for cases 2 and 3 are incorrect. No need for VALUES keyword when inserting values coming from a select statement.