Unique constraint to enforce max one null per item
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PostgreSQL multi-column unique constraint and NULL values
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How To Create A 'Two-Sided' Unique Index On Two Fields?
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Can a foreign key be NULL and/or duplicate?
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Why shouldn't we allow NULLs?
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Combinations of fields to be unique
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How do you scope uniquenuess based on two values with Rails 3 validations?
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SQL IN Clause 1000 item limit
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Is SOQL query with ID condition optimized to work as limit 1?
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SQL: how to select a single id ("row") that meets multiple criteria from a single column
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Foreign key constraints failures
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How can I manage the onslaught of null checks?
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What is the difference between Primary Key and unique key constraint?
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What to use, CASCADE DELETE rule or ON DELETE trigger for one to many constraint?
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constraint in oracle
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How to drop unique in MySQL?
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Add object constraint to a number of selected objects
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How to query records having at least one valid child record?