I have seen ::
in variety of places involving postgres code I have seen on the net. For example:
SELECT '{apple,cherry apple, avocado}'::text[];
It seems to be some sort of cast. What exactly is ::
in postgres and when should it be used?
I tried a bit of googling and searched the Postgres docs for ::
but got no good results.
I tried following searches in Google:
I tried the following searches in the postgres docs search button
This was almost embarrassing to ask on SO, but I figured Google will hopefully see this answer for other people in the future.
The type 'string' syntax is a generalization of the standard: SQL specifies this syntax only for a few data types, but PostgreSQL allows it for all types. The syntax with :: is historical PostgreSQL usage, as is the function-call syntax.
A type cast specifies a conversion from one data type to another.
PostgreSQL accepts two equivalent syntaxes for type casts, the PostgreSQL-specific value::type
and the SQL-standard CAST(value AS type)
.
In this specific case, '{apple,cherry apple, avocado}'::text[];
takes the string literal {apple,cherry apple, avocado}
and tells PostgreSQL to interpret it as an array of text
.
See the documentation on SQL expressions and arrays for details.
What @PSR and @Craig wrote.
Plus, there are two more syntax variants:
type value
This form only casts constants (string literals). Like in:
SELECT date '2013-03-21';
More in the manual in the chapter Constants of Other Types.
2.type(value)
That's the function-like syntax. Works only for types whose names are valid as function names. Like in:
SELECT date(date_as_text_col) FROM tbl;
More in the manual in the chapter Type Casts.
More comprehensive answer:
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