How can I find potential not null columns? - sql

Question

I'm working with a SQL Server database which is very light on constraints and want to apply some not null constraints. Is there any way to scan all nullable columns in the database and select which ones do not contain any nulls or even better count the number of null values?

Answer 1

Perhaps with a little dynamic SQL
Example
Declare #SQL varchar(max) = '>>>'
Select #SQL = #SQL
+ 'Union All Select TableName='''+quotename(Table_Schema)+'.'+quotename(Table_Name)+''''
+',ColumnName='''+quotename(Column_Name)+''''
+',NullValues=count(*)'
+' From '+quotename(Table_Schema)+'.'+quotename(Table_Name)
+' Where '+quotename(Column_Name)+' is null '
From INFORMATION_SCHEMA.COLUMNS
Where Is_Nullable='YES'
Select #SQL='Select * from (' + replace(#SQL,'>>>Union All ','') + ') A Where NullValues>0'
Exec(#SQL)
Returns (for example)
TableName ColumnName NullValues
[dbo].[OD-Map] [Map-Val2] 185
[dbo].[OD-Map] [Map-Val3] 225
[dbo].[OD-Map] [Map-Val4] 225
For all table/columns with counts >= 0
...
Select #SQL=replace(#SQL,'>>>Union All ','')
Exec(#SQL)

Answer 2

Check this query. This was originally written by Linda Lawton
Original Article: https://www.daimto.com/sql-server-finding-columns-with-null-values
Finding columns with null values in your Database - Find Nulls Script
set nocount on
declare #columnName nvarchar(500)
declare #tableName nvarchar(500)
declare #select nvarchar(500)
declare #sql nvarchar(500)
-- check if the Temp table already exists
if OBJECT_ID('tempdb..#LocalTempTable') is null
Begin
CREATE TABLE #LocalTempTable(
TableName varchar(150),
ColumnName varchar(150))
end
else
begin
truncate table #LocalTempTable;
end
-- Build a select for each of the columns in the database. That checks for nulls
DECLARE check_cursor CURSOR FOR
select column_name, table_name, concat(' Select ''',column_name,''',''',table_name,''' from ',table_name,' where [',COLUMN_NAME,'] is null')
from INFORMATION_SCHEMA.COLUMNS
OPEN check_cursor
FETCH NEXT FROM check_cursor
INTO #columnName, #tableName,#select
WHILE ##FETCH_STATUS = 0
BEGIN
-- Insert it if there if it exists.
set #sql = 'insert into #LocalTempTable (ColumnName, TableName)' + #select
print #sql
-- Run the statment
exec( #sql)
FETCH NEXT FROM check_cursor
INTO #columnName, #tableName,#select
end
CLOSE check_cursor;
DEALLOCATE check_cursor;
SELECT TableName, ColumnName, COUNT(TableName) 'Count'
FROM #LocalTempTable
GROUP BY TableName, ColumnName
ORDER BY TableName
The query result would be something like this.

Answer 3

This will tell you which columns in your database are currently NULLABLE.
USE <Your_DB_Name>
GO
SELECT o.name AS Table_Name
, c.name AS Column_Name
FROM sys.objects o
INNER JOIN sys.columns c ON o.object_id = c.object_id
AND c.is_nullable = 1 /* 1 = NULL, 0 = NOT NULL */
WHERE o.type_desc = 'USER_TABLE'
AND o.type NOT IN ('PK','F','D') /* NOT Primary, Foreign of Default Key */

Answer 4

Yes, it is fairly straight forward. Note: if the table contains a lot of records, I suggest using SELECT TOP 1000 *, instead of SELECT *.
-- Identify records where a specific column is NOT NULL
SELECT *
FROM TableName
WHERE ColumNName IS NOT NULL
-- Identify the count of records where a specific column contains NULL
SELECT COUNT(1)
FROM TableName
WHERE ColumNName IS NULL
-- Identify all NULLable columns in a database
SELECT *
FROM INFORMATION_SCHEMA.COLUMNS
WHERE IS_NULLABLE = 'YES'
For more information on the INFORMATION_SCHEMA views, see this: https://learn.microsoft.com/en-us/sql/relational-databases/system-information-schema-views/system-information-schema-views-transact-sql
If you want to scan all tables and columns in a given database for NULLs, then it is a two step process.
1.) Get the list of tables and columns that are NULLABLE.
-- Identify all NULLable columns in a database
SELECT TABLE_CATALOG, TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME
FROM INFORMATION_SCHEMA.COLUMNS
WHERE IS_NULLABLE = 'YES'
2.) Use Excel to create a SELECT statement to get the NULL counts for each table/column. To do this, copy and paste the query results from step 1 into EXCEL. Assuming you have copied the header row, then your data starts on row 2. In cell E2, enter the following formula.
="SELECT COUNT(1) FROM "&A2&"."&B2&"."&C2&" WHERE "&D2&" IS NULL"
Copy and paste that down the entire sheet. This will generate the SQL SELECT statement that you require. Copy the results in column E and paste into SQL Server and run it. This may take a while depending on the number of tables/columns to scan.