![]() ![]() ![]() This obviously prevents us from adding the same value more than once:ĮRROR: duplicate key value violates unique constraint "i"īut what about NULLs? NULL means undefined, so what should the unique index do when you try to insert more than one row with the value of NULL? Are two NULL values the same or not? If you ask PostgreSQL if NULL equals NULL, you get undefined (or NULL):Ĭomparing something which is not defined against something which is not defined does not make much sense. This will be a lot more efficient than to first change the source data. It is very important to understand that a NULL value is different from a zero value or a field that contains spaces. A field with a NULL value is a field with no value. A NULL value in a table is a value in a field that appears to be blank. Postgres=# create unique index i on t ( a ) The NULLIF will return NULL if the lat column is an empty string, otherwise it'll return the lat column value. NULLIF in postgreSQL is an in-built conditional function that takes two arguments and returns null if two arguments are equal. The PostgreSQL NULL is the term used to represent a missing value. What is it about? The answer is quite easy if we do a little example which shows the effect of this new feature.Ĭonsider the following table and unique index: For example, it is possible to add several columns and/or alter the type of several columns in a single command. So there is an obvious need to clarify what NULL is, how it works in different languages, and what the actual value is behind the NULL. All the forms of ALTER TABLE that act on a single table, except RENAME, SET SCHEMA, ATTACH PARTITION, and DETACH PARTITION can be combined into a list of multiple alterations to be applied together. ![]() Another nice feature was just committed for PostgreSQL 15: Add UNIQUE null treatment option (discussion here, commit details here). What is NULL There is often some confusion about NULL value in PostgreSQL, as it is treated differently in different languages. ![]()
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