unique constraint with a null column? - Mailing list pgsql-general

I have three columns, and one of them can be null. I'd
like to create a unique constraint across all three
columns and allow only one null value. e.g.

a|b|c
abc|123|null
abc|123|null # not allowed
abc|456|null
abc|456|987
abc|456|876
def|456|null
def|456|null # not allowed

Currently, the 'not allowed' lines are allowed.

thanks
csn





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