MySQL column shows NULL value when querying with WHERE clause despite containing actual data

[nateharris]

I’m working with a temporary table in a MySQL stored procedure and running into a weird issue. When I select all records without any conditions, the data looks perfect. But when I try to filter using a WHERE clause on a specific column, it acts like the values are NULL even though I can clearly see the data is there.

Here’s my temp table setup:

DROP TABLE IF EXISTS temp_records;
CREATE TEMPORARY TABLE temp_records (item_id BIGINT, status_code INT NOT NULL);

Inserting data like this:

INSERT IGNORE INTO temp_records (item_id, status_code) 
SELECT item_id, status_code FROM main_records;

When I check all data:

SELECT * FROM temp_records;

I get:

# item_id, status_code
'5', '200'

Looks good right? But then this query returns nothing:

SELECT * FROM temp_records WHERE status_code = 200;

And when I check again:

SELECT item_id, status_code FROM temp_records;

Now I see:

# item_id, status_code
5, null

I tried debugging with:

SELECT DISTINCT item_id, status_code,
       status_code IS NULL,
       status_code = '',
       CHAR_LENGTH(status_code),
       HEX(status_code)
FROM temp_records;

Result shows everything as NULL. The INSERT happens inside dynamic SQL within the stored procedure. Anyone know what could cause this behavior where data appears initially but then becomes NULL?

[John_Fast]

Had this exact problem - drove me crazy for hours. Your stored procedure’s probably losing data type context when it builds the dynamic SQL, especially with temp tables. MySQL gets confused about what type your variables actually are. Try casting your status_code explicitly: CAST(status_code AS SIGNED) in the INSERT, or just declare the column with a specific type upfront. Also check if you’ve got error handling that’s failing silently - INSERT IGNORE loves to hide conversion errors that mess up your data later. I’d throw a SELECT ROW_COUNT() right after the dynamic INSERT to make sure it’s actually inserting what you think it is.

[OwenNebula55]

This usually happens when your dynamic SQL has variable scope issues or transaction problems in the stored procedure. I’ve hit this exact issue before - the temp table gets created in a different session context than where you’re querying it. Check if your dynamic SQL runs in its own transaction that rolls back, or if there are implicit commits between your INSERT and SELECT. Also make sure your stored procedure isn’t dropping and recreating the temp table multiple times. Add some debugging output right after your INSERT to check the row count, and verify you’re not accidentally truncating the table somewhere in your procedure.

[Mike71]

sounds like maybe a character encoding issue or some hidden chars messing things up. try using TRIM() or look for odd whitespace with BINARY comparison. i’ve run into similar issues where data gets messed up in dynamic SQL inserts.