# Constrain insert on duplicate in other table

**URL:** https://forum.kirupa.com/t/constrain-insert-on-duplicate-in-other-table/294651
**Category:** programming
**Created:** [August 18, 2009, 4:29am UTC](https://forum.kirupa.com/t/constrain-insert-on-duplicate-in-other-table/294651 "2009-08-18T04:29:50Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![Fidodo](https://avatars.discourse-cdn.com/v4/letter/f/779978/32.png) [@Fidodo](https://forum.kirupa.com/u/Fidodo)
#### Post date: [August 18, 2009, 4:29am UTC](https://forum.kirupa.com/t/constrain-insert-on-duplicate-in-other-table/294651/1 "2009-08-18T04:29:50Z")

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I’m trying to figure out if there’s a way to do a unique check across multiple tables. I’m checking the users ip for a survey, and we’re creating a new one in which we don’t want people who took the first one to take. I’m wondering if it’s possible to do this in 1 statement? I know I can easily do this in 2 statements with a bit of server side logic, but I’m more curious about it in an educational way.

If MySQL were to support constraints would I be able to do a CHECK (count(table.ip) = 0), or would that not be possible since I’m referencing a different table? What would be the proper way to do that?

Thanks to anyone who knows!