Value Expressions

Value expressions are expressions which return a single value.

They can be used in many contexts of many statements.

A value expression can be:

  • A literal value
  • A column reference
  • A parameter reference
  • A operator invocation
  • A subscript expression
  • A function call
  • A type cast
  • An object constructor
  • An array constructor
  • A scalar subquery

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: true or false
  • string literals: 'this is a string literal'
  • numeric literals: 42

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: columnname
  • quoted: "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

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 = ?
  • $n as numbered placeholder: select * from t where x = $1 or x = $2

Operator Invocation

There are two different types of operators in CrateDB:

  • Binary: expression operator expression
  • Unary: operator expression

Subscript Expression

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.

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.

Function Call

A function is declared by its name followed by its arguments enclosed in 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 )

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 ... ] ']'

Scalar Subquery

A scalar subquery is a regular SELECT statement in parentheses that returns zero or one row with one column.

If zero rows are returned it is treated as null value. In case more than one row is returned it is an error.

Columns from relations from outside of the subquery cannot be accessed from within the subquery. Trying to do so will result in an error which states that the column is unknown.

Note

Scalar subqueries are restricted to SELECT statements and cannot be used in other statements.