How to detect and remove a column that contains only null values? - sql

Question

In my table table1 there are 6 columns Locations,a,b,c,d,e.
Locations [a] [b] [c] [d] [e]
[1] 10.00 Null Null 20.00 Null
[2] Null 30.00 Null Null Null
i need the result like
Locations [a] [b] [d]
[1] 10.00 Null 20.00
[2] Null 30.00 Null
My question is how to detect and delete column that contains all null values using sql query.
Is it possible?
If yes then please help and give sample.

Answer 1

Here is a fast (and ugly) stored proc that takes the name of the table and print (or drop if you want it to) the fields that are full of nulls.
ALTER procedure mysp_DropEmptyColumns
#tableName nvarchar(max)
as begin
declare #FieldName nvarchar(max)
declare #SQL nvarchar(max)
declare #CountDef nvarchar(max)
declare #FieldCount int
declare fieldNames cursor local fast_forward for
select c.name
from syscolumns c
inner join sysobjects o on c.id=o.id
where o.xtype='U'
and o.Name=#tableName
open fieldNames
fetch next from fieldNames into #FieldName
while (##fetch_status=0)
begin
set #SQL=N'select #Count=count(*) from "'+#TableName+'" where "'+#FieldName+'" is not null'
SET #CountDef = N'#Count int output';
exec sp_executeSQL #SQL, #CountDef, #Count = #FieldCount output
if (#FieldCount=0)
begin
set #SQL = 'alter table '+#TableName+' drop column '+#FieldName
/* exec sp_executeSQL #SQL */
print #SQL
end
fetch next from fieldNames into #FieldName
end
close fieldNames
end
This uses a cursor, and is a bit slow and convoluted, but I suspect that this is a kind of procedure that you'll be running often

Answer 2

How to detect whether a given column has only the NULL value:
SELECT 1 -- no GROUP BY therefore use a literal
FROM Locations
HAVING COUNT(a) = 0
AND COUNT(*) > 0;
The resultset will either consist of zero rows (column a has a non-NULL value) or one row (column a has only the NULL value). FWIW this code is Standard SQL-92.

Answer 3

SQL is more about working on rows rather than columns.
If you're talking about deleting rows where c is null, use:
delete from table1 where c is null
If you're talking about dropping a column when all rows have null for that column, I would just find a time where you could lock out the DB from users and execute one of:
select c from table1 group by c
select distinct c from table1
select count(c) from table1 where c is not null
Then, if you only get back just NULL (or 0 for that last one), weave your magic (the SQL Server command may be different):
alter table table1 drop column c
Do this for whatever columns you want.
You really need to be careful if you're deleting columns. Even though they may be full of nulls, there may be SQL queries out there that use that column. Dropping the column will break those queries.

Answer 4

SELECT * FROM table1 WHERE c IS NOT NULL -- or also SELECT COUNT(*)
To detect if indeed this column has no values at all.
ALTER TABLE table1 DROP COLUMN c
is the query to remove the column if it is deemed desirable.

Answer 5

Try this stored procedure with your table name as input.
alter proc USP_DropEmptyColumns
#TableName varchar(255)
as
begin
Declare #col varchar(255), #cmd varchar(max)
DECLARE getinfo cursor for
SELECT c.name FROM sys.tables t JOIN sys.columns c ON t.Object_ID = c.Object_ID
WHERE t.Name = #TableName
OPEN getinfo
FETCH NEXT FROM getinfo into #col
WHILE ##FETCH_STATUS = 0
BEGIN
SELECT #cmd = 'IF NOT EXISTS (SELECT top 1 * FROM [' + #TableName + '] WHERE [' + #col + '] IS NOT NULL)
BEGIN
ALTER TABLE [' + #TableName + '] DROP Column [' + #col + ']
end'
EXEC(#cmd)
FETCH NEXT FROM getinfo into #col
END
CLOSE getinfo
DEALLOCATE getinfo
end

Answer 6

PROC PRINT DATA=TABLE1;RUN;
PROC TRANSPOSE DATA=TABLE1 OUT=TRANS1;VAR A B C D E;RUN;
DATA TRANS2;SET TRANS1;IF COL1 = . AND COL2 = . THEN DELETE;RUN;
PROC TRANSPOSE DATA=TRANS2 OUT=TABLE2 (DROP=_NAME_);VAR COL1-COL2;RUN;
PROC PRINT DATA=TABLE2;RUN;

Answer 7

If you want to perform the stored proc on all the tables in your database.
DECLARE #table_name AS VARCHAR(128);
DECLARE table_cursor CURSOR FOR
SELECT name FROM sys.tables;
OPEN table_cursor;
FETCH NEXT FROM table_cursor INTO #table_name;
WHILE ##FETCH_STATUS = 0
BEGIN
EXEC USP_DropEmptyColumns #table_name;
FETCH NEXT FROM table_cursor INTO #table_name;
END;
CLOSE table_cursor;
DEALLOCATE table_cursor;