![]() ![]() 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. Postgres=# create unique index i on t ( a ) It is very important to understand that a NULL value is different from a zero value or a field that contains spaces. For example, 7 NULL yields null, as does 7 <> NULL. This PostgreSQL IS NULL example will insert records into the contacts table where the employeenumber contains a NULL value.A field with a NULL value is a field with no value. Ordinary comparison operators yield null (signifying unknown ), not true or false, when either input is null. A NULL value in a table is a value in a field that appears to be blank. ![]() In PostgreSQL, COALESCE() function allows to return an. The supported syntax is: COALESCE (value1,value2,value3. If the ELSE clause is omitted and no condition is true, the result is null. The NULL function can be used to provided alternate value of a column if it contains NULL value. If all the arguments are NULL then it will return NULL as its output. The COALESCE function returns the first non-NULL expression in the specified list. When one or more fields of a table needs to. Unknown values of the database table are treated differently by SQL. 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: The PostgreSQL NULL is the term used to represent a missing value. NULLIF, GREATEST, LEAST, and COALESCE are the types of conditional expressions in PostgreSQL. PostgreSQL: Using NULL and Not NULL clauses. ![]() Another nice feature was just committed for PostgreSQL 15: Add UNIQUE null treatment option (discussion here, commit details here). ![]()
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