Please help us improve Stack Overflow. Take our short survey. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Ask Question. Asked 9 years, 8 months ago. Active 4 years, 5 months ago. Viewed 26k times. It looks like some NULL values are appearing in the list.
Some NULL values are being filtered out by the query. I have checked. ID, g. Is your query posted verbatim? The first result isn't surprising. As a debug hint: Just output the PK so you see the row that generates the result. And while we are at it, you could post a screenshot of the entire ssms window so we can verify your claim more easily. I've created a connect item for this. I haven't tested to see if this issue occurs in SQL Server or not though. Show 31 more comments.
Active Oldest Votes. MicSim MicSim See stackoverflow. For text datatypes I've noticed before that updating to NULL doesn't update the bitmap but seems to set some sort of flag elsewhere in the pointer or pointed to structure. Great troubleshooting MicSim: you reduce the problem to a specific set of steps, reproduce the unwanted behavior, and find an explanation, congratulations!
Better go fix all the Text and Varchar max fields in our databases Add a comment. Both the inrow and out of row data remain unchanged. However at this point running the following incorrectly returns the row. Community Bot 1 1 1 silver badge. Martin Smith Martin Smith k 81 81 gold badges silver badges bronze badges.
Many thanks Martin. We'd have got here sooner if I'd had rights to do this on the server. We copied a backup down, but I didn't get time to try yesterday. Colin: I'm pretty sure all this is happening because of the database conversion. Could you run this and feedback us later with the results?
Best regards. I already know that I can turn numpties into nulls by running an update I ran the update above inside a transaction so I'm not sure that this will tell us anything more.
I wanted to keep the data in place to help with investigation, and the problem hasn't resulted in any significant issues yet. I don't have enough rights to view the underlying page data, so I think I will try to backup and pull the database file down onto a system where I do have admin rights. I built a 'holding' table on another drive, and populated it with the data from the source table, and wherever the fields were blank I used nullif [field],'' to insert nulls in place of the blanks.
Then I built a new table with exactly the same structure as the holding table, but instead of replacing blanks with null I just inserted the blanks, and so far it seems to be taking up more space I'm not yet done populating it and I can't be sure it is taking up more data yet. So before I fill it further and end up with a table bigger than I thought it would be, am I better off inserting nulls or blanks?
After migrating the data from the holding table to the new table, the new table is approximately 4gb larger. Note: For anyone wondering why it has apparently financial and numerical values stored in character fields: That was the original table design 17 years ago not by me and there are now hundreds of sql queries that run on this database, it is less work to keep them as varchar and the queries keep their casting, than it is to change them to money,int, or decimal and change hundreds of queries.
Repeated for a heap again, have to do multiple tests since we're guessing about your actual table structure :. So, negligible, as I suggested, even extrapolating over ,, rows, the biggest possible difference once more, depending on your schema would be KB on a proper table, and 6. If your server is so tight on disk space that 6.
Basically, a variable width column varchar stores a bitmap that indicates null or not null. If it's null, then zero bytes are allocated for the varchar field and the bit gets flipped. For fixed width columns char , the entire field is still allocated, with no data stored in it. So a yte char field will allocate 10 bytes, NULL or not. That article does an insert with data, with NULL, and with empty string. It then polls the page size to see what's going on internally.
For at least the standard FixedVar record format it does not make any difference to how much space the table consumes it might make a marginal difference to indexes as discussed later. Both a null varchar and an empty string are stored in exactly the same way. The only way they are distinguished is whether there was a 1 or 0 in the null bitmap. They both take zero length in the variable length column data section and both can also avoid taking up two bytes in the variable column offset array if they are not followed by any columns that do contain data.
This is not true for datapages. See Myth 6b: The null bitmap only contains bits for nullable columns. There is a slight difference for indexes in that if all the columns participating in an index are not nullable the null bitmap is omitted.
However the difference is negligible and you should be choosing the option that gives you the desired semantics. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Learn more. Asked 4 years, 11 months ago. Active 4 years, 11 months ago. Viewed 17k times. I realize this may be marked as duplicate, but I'm specifically asking in relation to SQL Server I have read conflicting advice on the internet so I'm asking here. Then I built a new table with exactly the same structure as the holding table, but instead of replacing blanks with null I just inserted the blanks, and so far it seems to be taking up more space I'm not yet done populating it and I can't be sure it is taking up more data yet So before I fill it further and end up with a table bigger than I thought it would be, am I better off inserting nulls or blanks?
Edit: After migrating the data from the holding table to the new table, the new table is approximately 4gb larger. Improve this question. MrVimes MrVimes 1 1 gold badge 8 8 silver badges 17 17 bronze badges. The space difference here will be negligible if any share DDL. The more important factor is semantics. Does "blank space" mean absence of value? What if an empty string later becomes to mean something other than that?
NULL is a useful thing; don't fear it. My problem is it's not negligible on a table with million rows on an old server with limited disk space. Negligible suggests there is a difference. It seems you are fully capable of running a test to determine which of those articles to believe. So if your concern is only about space, not sure why you're asking.
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