jrara jrara - 4 months ago 9
SQL Question

How to find which columns don't have any data (all values are NULL)?

I have several tables in a database. I would like to find which columns (in which tables) don't have any values (all NULL in a column). I the example below, the result should be

TestTable1 --> Var2
TestTable2 --> Variable1


I don't have any idea how to create this kind of query. Your help is most appreciated!

--create first table
create table dbo.TestTable1 (
sur_id int identity(1,1) not null primary key,
var1 int null,
var2 int null
)
go

--insert some values
insert into dbo.TestTable1 (var1)
select 1 union all select 2 union all select 3

--create second table
create table dbo.TestTable2 (
sur_id int identity(1,1) not null primary key,
variable1 int null,
variable2 int null
)

--and insert some values
insert into dbo.TestTable2 (variable2)
select 1 union all select 2 union all select 3

Answer

For a single column, count(ColumnName) returns the number of rows where ColumName is not null:

select  count(TheColumn)
from    YourTable

You can generate a query for all columns. Per Martin's suggestion, you can exclude columns that cannot be null with is_nullable = 1. For example:

select  'count(' + name + ') as ' + name + ', '
from    sys.columns
where   object_id = object_id('YourTable')
        and is_nullable = 1

If the number of tables is large, you can generate a query for all tables in a similiar way. The list of all tables is in sys.tables.