Select 某些列仅在满足条件时

Question

I want to have a select that will not have fixed number of columns. Column OptionalColumn should be selected only if variable @check = 1

This is what I had (syntax is bad but maybe it explains my problem)

SELECT 
    Column1, Column2,
    (IF @Check = 1
       BEGIN OptionalColumn END)
FROM table

It is crucial to have it only if @Check = 1 and not having it if @Check = 0. Is it doable?

Answer 1

You could do this with an IF ELSE block like this;

CREATE TABLE #table (Column1 varchar(10), Column2 varchar(10))
INSERT INTO #table
VALUES
('column1', 'column2')

DECLARE @Check bit; SET @Check = 0

IF @Check = 1 
(SELECT Column1, Column2
FROM #table)

ELSE

(SELECT Column1
FROM #table)

Change the variable from 0 to 1 for testing to see the change in number of columns.

Answer 2

Rich Benner gave you a good solution, as an alternative you can use dynamic sql like that :

DECLARE  @sqlvar VARCHAR(100)
select @Sqlvar= ' SELECT Column1, Column2 '+IIF (@check=1,',OptionalColumn','')+' from TABLE';
EXECUTE sp_executesql @sqlvar 

For reference on sp_ executesql look: [https://msdn.microsoft.com/it-it/library/ms188001(v=sql.120).aspx]

Answer 3

Can use IF ELSE block. Please try this.

--Creating a Table.
CREATE TABLE #MyTable 
    (
    Column1 VARCHAR(10), 
    Column2 VARCHAR(10),
    OptionalColumn VARCHAR(10)
    )
--Inserting value to the Table.
INSERT INTO #MyTable
VALUES('Value 1', 'Value 1','Optional Value')

--IF ELSE Logic to desired output.
DECLARE @Check bit 
SET @Check = 0

IF @Check = 1 
    (
    SELECT 
        Column1, 
        Column2
    FROM 
        #MyTable
    )
ELSE
   (
    SELECT 
        Column1, 
        Column2,
        OptionalColumn
    FROM 
        #MyTable
    )

Answer 4

I find that it's pretty clean and readable to use temp tables for this purpose, and it doesn't require duplicating queries or big branches of logic.

Create a temp table with all the columns you would ever need. Use IF statements to drop the columns you don't want to display from the temp table. Then finally, select * from the temp table.

This will also allow you use the data returned to make a determination on whether to keep the column or not.

 IF OBJECT_ID('tempdb..#tempTable') IS NOT NULL DROP TABLE #tempTable;
 select '---' as [1],
 '---' as [2],
 '---' as [3],
 '---' as [4],
 '---' as [5],
 '---' as [6],
 '---' as [7],
 null as [8]
 into #tempTable 

if RAND() > .5
begin
    alter table #tempTable drop column [1]
end

if RAND() > .5
begin
    alter table #tempTable drop column [2]
end

if RAND() > .5
begin
    alter table #tempTable drop column [3]
end

--Query using result set to dertmine if column should be dropped
if (select count(*) from #tempTable where [8] is not null) = 0
begin
    alter table #tempTable drop column [8]
end

select * from  #tempTable