Expression Notation (CXN)
Expressions in CDS definitions and queries can be one of:
expr = // one of...
val | // [literal values]: #literal-values
ref | // references or functions
xpr | // operator expressions
func | // function calls
param | // binding parameters
sym | // enum symbol
SELECT // subqueries
Literal Values
Literal values are represented as {val:...} with property val holding the actual
literal value as specified in JSON.
val = {val:literal}
literal = string | number | true | false | null
Examples:
cds.parse.expr(`'a string'`) == {val:'a string'}
cds.parse.expr(`11`) == {val:11}
cds.parse.expr(`true`) == {val:true}
cds.parse.expr(`null`) == {val:null}
References
A reference is represented as {ref:...} with property ref holding an array of reference segments as plain identifier strings, or in case of infix filters and/or arguments an object {id:identifier, ...} as follows:
ref = {ref:[..._segment]}
_segment = string | { id:string, args:_named, where:expr }
_named = { ... <name>:expr }
Examples:
let cqn4 = cds.parse.expr
cqn4(`foo.bar`) == {ref:['foo','bar']}
cqn4(`foo[9].bar`) == {ref:[{ id:'foo', where:[{val:9}] }, 'bar' ]}
cqn4(`foo(p:x).bar`) == {ref:[{ id:'foo', args:{p:{ref:['x']}} }, 'bar' ]}
Function Calls
Function calls are represented as follows:
func = { func:string, args: _positional | _named }
_positional = [ ...expr ]
_named = { ... <name>:expr }
Examples:
let cqn4 = cds.parse.expr
cqn4(`foo(p=>x)`) == {func:'foo', args:{p:{ref:['x']}}}
cqn4(`sum(x)`) == {func:'sum', args:[{ref:['x']}]}
cqn4(`count(*)`) == {func:'count', args:['*']}
Operator Expressions
Operators join one or more expressions into complex ones, represented
as {xpr:...}. The property xpr holds a sequence of operators and operands.
xpr = {xpr:_xpr}
_xpr = [...( _operand | _operator )]
_operand = expr
_operator = string
- Operands can be any kind of expression
- Operators are represented as plain strings, like
'='or'and'
Examples:
cds.parse.expr(`x<9`) ==//> returns:
{xpr:[ {ref:['x']}, '<', {val:9} ]}
cds.parse.expr(`x<9 and (y=1 or z=2)`) ==//> returns:
{xpr:[
{ref:['x']}, '<', {val:9}, 'and', '(',
{ref:['y']}, '=', {val:1}, 'or', {ref:['z']}, '=', {val:2}
')'
]}
CQN intentionally doesn’t aim to understand the individual operators and related expressions. It captures them as arbitrary sequences, in the same lexical structure and order they’re written in the source. This ‘ignorance’ allows us to stay open to any kind of operators and keywords. For example, we can easily express native extensions of underlying database dialects.
Binding Parameters
Binding parameters for prepared statements are represented as {ref:..., param:true} with values for ref as follows.
param = { ref:[ '?' | number | name ], param:true }
Examples:
cds.parse.expr(`x=:1`) == [{ref:['x']}, '=', {ref:[1], param:true}]
cds.parse.expr(`x=:y`) == [{ref:['x']}, '=', {ref:['y'], param:true}]
cds.parse.expr(`x=?`) == [{ref:['x']}, '=', {ref:['?'], param:true}]