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Johnny NewbeeFlag for United States of America asked on

Mysql delete records


I have about twenty tables in Mysql 5.0 db for a application. For demo purpose, I need to delete the most records except three rows left, which should have linked records among the tables. What should be the correct procedure to generate proper linkage and a complete delete sql that only leave three rows left?

Thank you.

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Johnny Newbee

8/22/2022 - Mon
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delete from table1 where id in (select id from ... your criteria ... limit 3, 100000)
Johnny Newbee


Thanks for the quick response. But please read my question again clearly. I need a sql to delete most records in all tables once. And the 3 records left should be linked among the tables.

Mark Wills

The danger there is not so much removing records, but making sure the records remaining still have referential integrity across the database so the remaining demo data does make sense and does display...

So, you will need to select a few customers, products, or people, (or whatever the dtabase represents), and retain sufficient data to best represent the capabilities that you need for your demo database (possibly including some history).

You would then remove any data elements that did not contribute or belong to the chosen "master" / "header" records.

for headers / masters it would be something like:

delete from hdr_table where key not in ('KeepThis','AndThis','AndthisOneAswell')

then for details / transaction data :

delete from dtl_table where not exists (select key from hdr_table)

does that make sense ?
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James Murphy
Mark Wills

sorry, that delete should be : delete from dtl_table where not exists (select * from hdr_table where dtl_table.key = hdr_table.key)
Johnny Newbee

Hi Mark,

I've seen you are on the right track. Here are my follow up to your response:

1) How do I make sure referential integrity is there? If not, is some kind of tool I could use with Mysql to make that happen automatically or semi-auto?

2) delete from dtl_table where not exists (select * from hdr_table where dtl_table.key = hdr_table.key) it looks like I am still deleting one table at one time. No way I could do it once for all tables?

Thank you.

Mark Wills

I am not aware of any off the top of my head. You would need to publish a comprehension data dictionary and data model for it to work properly. and by the time you have done that...

If you have referential integrity with foreign keys etc throughout your system, then cascading deletes are possible - depending on how you set up foreign keys.

otherwise, it is a search of the schema, get the keys, and start deleting...

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Johnny Newbee

Thanks for the referential guide line. But the sql has to delete one table by one time?
Mark Wills

Mark Wills

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Johnny Newbee

Too bad to hear that.

FYI, here is the db tool I just found new on the street:


Hope you could find it useful too.
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Johnny Newbee

It could be a complete solution if I don't have to manually delete records from 20+ tables; or if there is a such sql in the future :)