SQL Projections

A projection is a value that is returned by a query statement (SELECT, MATCH).

Eg. the following query

SELECT name as firstName, age * 12 as ageInMonths, out("Friend") from Person where surname = 'Smith'

has three projections:

  • name as firstName
  • age * 12 as ageInMonths
  • out("Friend")

Syntax

A projection has the following syntax:

<expression> [<nestedProjection>] [ AS <alias> ]

  • <expression> is an expression (see SQL Syntax) that represents the way to calculate the value of the single projection
  • <alias> is the Identifier (see SQL Syntax) representing the field name used to return the value in the result set

A projection block has the following syntax:

[DISTINCT] <projection> [, <projection> ]*

  • DISTINCT: removes duplicates from the result-set

IMPORTANT - the old distinct() function (used in OrientDB <=2.x) is no longer supported

Query result

By default, a query returns a different result-set based on the projections it has:

  • * alone: The result set is made of records as they arrive from the target, with the original @rid and @class attributes (if any)
  • * plus other projections: records of the original target, merged with the other projection values, with @rid, @class and @version of the original record.
  • no projections: same behavior as *
  • expand(<projection>): The result set is made of the records returned by the projection, expanded (if the projection result is a link or a colleciton of links) and unwinded (if the projection result is a collection). Nothing in all the other cases.
  • one or more projections: temporary records (with temporary @rid and no @class). Projections that represent links are returned as simple @rid values, unless differently specified in the fetchplan.

IMPORTANT - projection values can be overwritten in the final result, the overwrite happens from left to right

eg.

SELECT 1 as a, 2 as a

will return [{"@rid": "-2:0", "a":2}]

eg.

Having the record {"@class":"Foo", "name":"bar", "@rid":"#12:0", "@version": 2}

SELECT *, "hey" as name from Foo

will return [{"@class":"Foo", "@rid":"#12:0", "@version": 2, "name":"hey"}]

SELECT  "hey" as name, * from Foo

will return [{"@class":"Foo", "@rid":"#12:0", "@version": 2, "name":"bar"}]

ATTENTION: when saving back a record with a valid rid, you will overwrite the existing record! So pay attention when using * together with other projections

IMPORTANT - the result of the query can be further unwound using the UNWIND operator

IMPORTANT: expand() cannot be used together with GROUP BY

Aliases

The alias is the field name that a projection will have in the result-set.

An alias can be implicit, if declared with the AS keyword, eg.

SELECT name + " " + surname as full_name from Person

result:
[{"@rid":"#-2:0", "full_name":"John Smith"}]

An alias can be implicit, when no AS is defined, eg.

SELECT name from Person

result:
[{"@rid":"#-2:0", "name":"John"}]

An implicit alias is calculated based on how the projection is written. By default, OrientDB uses the plain String representation of the projection as alias.

SELECT 1+2 as sum

result:
[{"@rid":"#-2:0", "sum": 3}]
SELECT parent.name+" "+parent.surname as full_name from Node

result:
[{"@rid":"#-2:0", "full_name": "John Smith"}]

The String representation of a projection is the exact representation of the projection string, without spaces before and after dots and brackets, no spaces before commans, a single space before and after operators.

eg.

SELECT 1+2 

result:
[{"@rid":"#-2:0", "1 + 2": 3}] /* see the space before and after the + sign */
SELECT parent.name+" "+parent.surname from Node

result:
[{"@rid":"#-2:0", "parent.name + \" \" + parent.nurname": "John Smith"}]
SELECT items[4] from Node

result:
[{"@rid":"#-2:0", "items[4]": "John Smith"}]

Nested projections

(since v 3.0 M2)

Syntax:

":{" ( * | (["!"] <identifier> ["*"] (<comma> ["!"] <identifier> ["*"])* ) ) "}"

A projection can refer to a link or to a collection of links, eg. a LINKLIST or a LINKSET. In some cases you can be interested in the expanded object intead of the RID.

Let's clarify this with an example. This is our dataset:

@rid name surname parent
#12:0 foo fooz
#12:1 bar barz #12:0
#12:2 baz bazz #12:1

Given this query:

SELECT name, parent FROM TheClass WHERE name = 'baz'

The result is

{ 
   "name": "baz",
   "parent": #12:1
}

Now suppose you want to expand the link and retrieve some properties of the linked object. You can do it explicitly do it with other projections:

SELECT name, parent.name FROM TheClass WHERE name = 'baz'
{ 
   "name": "baz",
   "parent.name": "bar"
}

but this will force you to list them one by one, and it's not always possible, especially when you don't know all their names.

Another alternative is to use nested projections, eg.

SELECT name, parent:{name} FROM TheClass WHERE name = 'baz'
{ 
   "name": "baz",
   "parent": {
      "name": "bar"
   }
}

or with multiple attributes

SELECT name, parent:{name, surname} FROM TheClass WHERE name = 'baz'
{ 
   "name": "baz",
   "parent": {
      "name": "bar"
      "surname": "barz"      
   }
}

or using a wildcard

SELECT name, parent:{*} FROM TheClass WHERE name = 'baz'
{ 
   "name": "baz",
   "parent": {
      "name": "bar"
      "surname": "barz"      
      "parent": #12:0
   }
}

You can also use the ! exclude syntax to define which attributes you want to exclude from the nested projection:

SELECT name, parent:{!surname} FROM TheClass WHERE name = 'baz'
{ 
   "name": "baz",
   "parent": {
      "name": "bar"
      "parent": #12:0
   }
}

You can also use a wildcard on the right of property names, to specify the inclusion of attributes that start with a prefix, eg.

SELECT name, parent:{surna*} FROM TheClass WHERE name = 'baz'
{ 
   "name": "baz",
   "parent": {
      "surname": "barz"      
   }
}

or their exclusion

SELECT name, parent:{!surna*} FROM TheClass WHERE name = 'baz'
{ 
   "name": "baz",
   "parent": {
      "name": "bar",      
      "parent": #12:0
   }
}

Nested projection syntax allows for multiple level depth expressions, eg. you can go three levels deep as follows:

SELECT name, parent:{name, surname, parent:{name, surname}} FROM TheClass WHERE name = 'baz'
{ 
   "name": "baz",
   "parent": {
      "name": "bar"
      "surname": "barz"      
      "parent": {
         "name": "foo"
         "surname": "fooz"      
      }   
   }
}

You can also use expressions and aliases in nested projections:

SELECT name, parent.parent:{name, surname} as grandparent FROM TheClass WHERE name = 'baz'
{ 
   "name": "baz",
   "grandparent": {
      "name": "foo"
      "surname": "fooz"      
   }   
}

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