How do I CAST a date in a subquery? Currently getting an error: Conversion failed when converting date and/or time from character string

All we need is an easy explanation of the problem, so here it is.

I have a column called Source in my table called TPR. The Source column is of datatype varchar and contains text like January 2020 and Q3 Completed. When I run a query that only extracts the date-like fields to CAST as dates like so:

SELECT CAST(Source AS DATE) AS Source
FROM TPR
WHERE Source NOT LIKE 'Q3 Completed`

I get the appropriate results. When I place the query above in a subquery to act as a filter like so:

SELECT CaseNumber
FROM TPR
WHERE Source =
     SELECT CAST(Source AS DATE) AS Source
     FROM TPR
     WHERE Source NOT LIKE 'Q3 Completed`

I get the following error:

Conversion failed when converting date and/or time from character string.
Warning: Null value is eliminated by an aggregate or other SET operation.

I’ve also tried the following but it too failed:

SELECT CaseNumber
FROM TPR
WHERE CAST(Source AS DATE) =
     SELECT CAST(Source AS DATE) AS Source
     FROM TPR
     WHERE Source NOT LIKE 'Q3 Completed`

I don’t understand why this is failing.

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

Use a CTE to get the wanted rows and then do the conversion

If the strings are not a correct format like seen below, you will get an error.

CREATE TABLE TPR(CaseNumber int,Source varchar(100))
GO

INSERT INTO TPR VALUES (1,'Q3 Completed'),(2,'2021/01/01')
GO

WITH CTE AS (
     SELECT Source
     FROM TPR
     WHERE Source NOT LIKE 'Q3 Completed' )
     SELECT CaseNumber
FROM TPR
WHERE Source IN (SELECT Source FROM CTE)
GO

| CaseNumber |
| ---------: |
|          2 |

db<>fiddle here

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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