SQL omit columns with all same value - sql

Question

Is there a way to write a SQL query to omit columns that have all of the same values? For example,
row A B
1 9 0
2 7 0
3 5 0
4 2 0
I'd like to return just
row A
1 9
2 7
3 5
4 2

Answer 1

Although it is possible to use SQL to find if all rows in a column have identical values, there is no way to make a fixed SQL statement not return a column based on the content of the query.
Here is how to find out if all items in a column have identical values:
SELECT COUNT(row)=COUNT(DISTINCT B) FROM my_table
You can run a preliminary query to see if a column needs to be displayed, and then form the query dynamically, including the column only when you need it.

Answer 2

The only way to change what columns are returned is by executing separate queries. So you'd have to do something like:
IF EXISTS(SELECT null FROM myTable WHERE B <> 0)
BEGIN
SELECT row, A, B FROM myTable
END
ELSE
BEGIN
SELECT row, A FROM myTable
END
However it's generally bad practice to return different columns based on the data - otherwise you make the client determine if a particular column is in the result set first before trying to access the data.
This sort of requirement is more commonly done when displaying the data, e.g. in a web page, in a report, etc.

Answer 3

There is no way to write a query statement that will only return columns that have disparate values. You can however use some conditonnal statements to execute different queries based on your needs.
You could also insert your query result into a temporary table, loop over the columns, build a new select statement that includes only the columns that have different values and finally execute the statement.
Note: You should probably just include bit columns to indicate wheter columns are all duplicates or not. The application could then just discard any column that has been indicated as all duplicates.
Anyway, here's an example solution for SQL SERVER
-- insert results into a temp table
SELECT *
INTO #data
FROM (
SELECT 1 AS col1, 1 AS col2, 2 AS col3
UNION ALL
SELECT 2, 1, 3
) d;
DECLARE
#column sysname,
#sql nvarchar(max) = '',
#finalSql nvarchar(500) = 'SELECT ',
#allDuplicates bit;
DECLARE colsCur CURSOR
FOR
SELECT name
FROM tempdb.sys.columns
WHERE object_id = OBJECT_ID('tempdb..#data');
OPEN colsCur;
FETCH NEXT FROM colsCur INTO #column;
WHILE ##FETCH_STATUS = 0
BEGIN
SET #sql = N'SELECT #allDuplicates = CASE COUNT(DISTINCT ' + #column + ') WHEN 1 THEN 1 ELSE 0 END FROM #data';
EXEC sp_executesql #sql, N'#allDuplicates int OUT', #allDuplicates = #allDuplicates OUT;
IF #allDuplicates = 0 SET #finalSql = #finalSql + #column + ',';
FETCH NEXT FROM colsCur INTO #column;
END
CLOSE colsCur;
DEALLOCATE colsCur;
SET #finalSql = LEFT(#finalSql, LEN(#finalSql) - 1) + ' FROM #data';
EXEC sp_executesql #finalSql;
DROP TABLE #data;