NOT LIKE and LIKE not returning opposite result

Question

I have a table with 200 records out of which 10 records has text containing the word 'TAX'.

When I'm executing

Select * from tbl1 WHERE [TextCol] LIKE '%TAX%'

then I get the result set with those 10 records correctly .

But when I am trying to exclude those records by

Select * from tbl1 WHERE [TextCol] NOT LIKE '%TAX%'

it's returning 100 records only, instead of 190.

Answer 1

1. Select * from tbl1 
   WHERE ([TextCol] NOT LIKE '%TAX%') AND ([TextCol] NOT LIKE '%TAX%')
   

2. select * from tbl1
   where [TextCol] NOT LIKE '%TAX%' OR [TextCol] IS NULL
   

Answer 2

(A) SQL comparison operators result in three possible values: True, False and Unknown. If one or both operands are NULL then the result is Unknown. Consider the following example where we compare some values (a person's age) with a constant (18):

21   >= 18 -- True
15   >= 18 -- False
NULL >= 18 -- Unknown

As you can see, the database can/will not decide if NULL is greater than/equal to 18.

(B) The database will only return rows where the WHERE clause evaluates to True. Inverting the expression (e.g. WHERE age >= 18 changed to WHERE age < 18) does not affect Unknown results.

You can use the IS [NOT] NULL to match NULL values. The following query will select the rows where the column does not match the pattern OR the column is NULL:

WHERE [TextCol] NOT LIKE '%TAX%' OR [TextCol] IS NULL

Functions such as ISNULL and COALESCE can be used to transform NULL into some value.

Answer 3

I had the same problem with the IN operator on simple int column with nulls. I found that these where not each others inverse as I thought. (I could tell by the row count)

select * from Dest where id in(select id from Source)
select * from Dest where id NOT in(select id from Source)

To get each others invert I had too rewrite them as such:

select * from Dest where isnull(id,-2)  in(select isnull(id,-1) from Source) 
select * from Dest where isnull(id,-2) NOT in(select isnull(id,-1) from Source) 

Answer 4

You need to check for NULL values as well:

    [TextCol] NOT LIKE '%TAX%' OR [TextCol] IS NULL

This should take care of null values as well , which is probably why you didn't get all the rows in the output .

Answer 5

Does this return the correct result ?

Select * from tbl1 WHERE COALESCE([TextCol],'-1') NOT LIKE '%TAX%'

I believe NULL values are the issue here, if the column contains them, then NULL NOT LIKE '%TAX%' will return UNKNOWN/NULL and therefore won't be selected.

I advise you to read about handling with NULL values , or here.

As @ughai suggested, if performance is an issue you can also use:

  Select * from tbl1 
  WHERE [TextCol] NOT LIKE '%TAX%'
     OR [TextCol] IS NULL

Answer 6

It happen to me once too! after breaking my head around it I found out it was because of null values, so you can use this query to avoid it:

WHERE CASE WHEN [TextCol] IS NULL
           THEN 'default' 
           ELSE [TextCol] 
      END NOT LIKE '%TAX%'