CASE

The CASE expression is a conditional expression that allows you to evaluate one or more conditions and return different results based on those conditions.

Syntax
<case expression> ::= <simple case> | <general case>

GQL supports two forms of the CASE expression:

• Simple CASE
• General CASE

Example Graph

GQL
INSERT (p1:Paper {_id:'P1', title:'Efficient Graph Search', score:6, author:'Alex', publisher:'PulsePress'}),
       (p2:Paper {_id:'P2', title:'Optimizing Queries', score:9, author:'Alex'}),
       (p3:Paper {_id:'P3', title:'Path Patterns', score:7, author:'Zack', publisher:'BrightLeaf'}),
       (p1)-[:Cites {weight:2}]->(p2),
       (p2)-[:Cites {weight:1}]->(p3)

Simple CASE

The simple CASE expression evaluates a single expression against multiple possible values, returning the result associated with the first matching value.

Syntax
<simple case> ::=
  "CASE" <expr>
    { "WHEN" <value> "THEN" <expr> }...
    [ "ELSE" <expr> ]
  "END"

Details

• The <expr> is evaluated once, then compared for equality against each WHEN value in order.
• The first match returns the corresponding THEN value.
• If no match is found, returns the ELSE value. If ELSE is omitted, null is returned.
• Only equality comparison is supported. For conditions involving operators (<, >, IS NULL, etc.), use General CASE.

GQL
MATCH (n:Paper WHERE n.score > 6)
RETURN CASE count(n) WHEN 3 THEN "Y" ELSE "N" END AS result

Result: "N"

GQL
MATCH (n:Paper)
RETURN n.title, n.score,
CASE n.score
  WHEN 6 THEN "Low"
  WHEN 7 THEN "Medium"
  WHEN 8 THEN "Medium"
ELSE "High" END AS scoreLevel

Result:

General CASE

The general CASE expression evaluates multiple conditions, returning the result associated with the first condition that evaluates to true.

Syntax
<general case> ::=
  "CASE"
    { "WHEN" <condition> "THEN" <expr> }...
    [ "ELSE" <expr> ]
  "END"

Details

• Each <condition> is a boolean expression evaluated sequentially.
• The first condition that evaluates to true returns the corresponding THEN value.
• If no condition is true, returns the ELSE value. If ELSE is omitted, null is returned.

GQL
MATCH (n:Paper)
RETURN n.title,
CASE
  WHEN n.publisher IS NULL THEN "Publisher N/A"
  WHEN n.score < 7 THEN -1
  ELSE n.author
END AS note

Result:

GQL
MATCH (n:Paper)
RETURN n.title, n.score,
CASE
  WHEN n.score < 7 THEN "Low"
  WHEN n.score <= 8 THEN "Medium"
  ELSE "High"
END AS scoreLevel

Result: