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18 lines
675 B
Markdown
18 lines
675 B
Markdown
# Creating Conditional Constraints
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There are times when it doesn't make sense for a constraint to apply to all
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records in a table. For instance, if we have a table of pokemon, we may only
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want to apply a unique index constraint to the names of non-wild pokemon.
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This can be achieved with the following conditional constraint:
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```sql
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create unique index pokemons_names on pokemons (names)
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where wild = false;
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```
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If we try to insert a non-wild pokemon with a duplicate name, we will get an
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error. Likewise, if we try to update a pokemon with a duplicate name from
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wild to non-wild, we will get an error.
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[source](http://www.postgresguide.com/performance/conditional.html)
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