Value expressions¶
A value expression is a combination of one or more values, operators, and functions that evaluate to a single value.
Table of contents
Literal value¶
A literal is a notation to represent a value within a statement.
Different types have different notations. The simplest forms are:
boolean literals:
trueorfalsestring literals:
'this is a string literal'numeric literals:
42interval literals:
INTERVAL '1' SECOND
See also
Column reference¶
A column reference is the name of a column. It’s represented using an identifier. An identifier is an unquoted or double quoted string.
unquoted:
columnnamequoted:
"columnName"
It’s also possible to include the name of a table or alias in order to unambiguously identify a column of a specific relation if a statement contains multiple alias or table definitions:
tab0.columnname
See also
Parameter reference¶
A parameter reference is a placeholder for a value.
CrateDB clients usually have some kind of API to provide those values.
Parameter references can either be unnumbered or numbered:
Question mark as an unnumbered placeholder:
select * from t where x = ?$nas numbered placeholder:select * from t where x = $1 or x = $2
Operator invocation¶
An operator can be invoked as a value expression in one of two ways: binary or unary.
The syntax of a binary operator:
expression operator expression
The syntax of a unary operator:
operator expression
Subscripts¶
A subscript expression is an expression which contains a subscript
operator ([ ]). It can be used to access a sub
value of a composite type value.
Array subscript¶
The subscript operator can be used on array expressions to retrieve a single element of an array:
array_expression[ array_index ]
array_index is a 1 based integer specifying the position of the element in
the array which should be retrieved.
See also
Object subscript¶
On object expressions the subscript operator can be used to access an inner element of the object:
obj_expression['key']
The key must be a string literal which is the name of the element which
should be retrieved.
See also
Record subscript¶
Record subscript retrieves the value of a field within a record or object. This is similar to object subscripts.
Synopsis:
(record_expression).fieldName
Example:
cr> SELECT (information_schema._pg_expandarray(ARRAY['a', 'b'])).n AS n
+---+
| n |
+---+
| 1 |
| 2 |
+---+
SELECT 2 rows in set (... sec)
expression is an expression of type record or object and key is
an identifier that must refer to a field of the record.
Function call¶
A function can be invoked with a function call (a process better known as calling the function). The corresponding syntax is the function name optionally followed by zero or more arguments (in the form of value expressions) enclosed by parentheses:
function_name[([expression [, expression ... ]])]
Type cast¶
A type cast specifies the conversion from one type to another. The syntax is:
CAST(expression as type)
Another variant to do type casts is try_cast. Instead of raising an error
this returns null if a value cannot be converted to the given type:
TRY_CAST(expression as type)
See also
Object constructor¶
A object constructor is an expression which builds an object using its arguments.
It consists of one ore more ident = expression, separated by commas and
enclosed in curly brackets:
{ elementNameIdent = valueExpression [, elementNameIdent = valueExpression ...] }
See also
Array constructor¶
A array constructor is an expression which builds an array. It consists of one
or more expressions separated by commas, enclosed in square brackets and
optionally prefixed with ARRAY:
[ ARRAY ] '[' expression [, expression ... ] ']'
See also
Another way to construct an array is by using an ARRAY(subquery) expression
as part of the SELECT list of a SELECT
statement:
ARRAY '(' subquery ')'
Example:
cr> select array(select height from sys.summits order by height desc limit 5)
... as top5_mountains_array;
+--------------------------------+
| top5_mountains_array |
+--------------------------------+
| [4808, 4634, 4545, 4527, 4506] |
+--------------------------------+
SELECT 1 row in set (... sec)
Note
Array constructor only supports subqueries returning a single column.
Scalar subquery¶
A scalar subquery (also known as a subquery expression) is a subquery that returns a single value (i.e., one row with one column).
If zero rows are returned, it will be treated as null value. In the case that more than one row (or more than one column) is returned, CrateDB will treat it as an error.
Columns from relations from outside of the subquery cannot be accessed from within the subquery. If you try to do so, CrateDB will treat it as an error, stating that the column is unknown.