Source: https://dev-docs.nebula-graph.io/v5.3.2/database-gql-reference/fe/expressions/case/
Documentation version: v5.3.2

# Case value expression

The case value expression returns a value based on the evaluation of one or more search conditions.

The system evaluates the `WHEN` conditions one by one in the order in which they are written. Once a condition is evaluated as true, the system returns the value from the corresponding `THEN` clause. If none of the conditions are true, the system returns the value from the `ELSE` clause. If no `ELSE` clause is specified, the system returns `NULL`.

In GQL, the `coalesce()` function is equivalent to the case value expression. This function returns the first non-null value in a list value expression. If the values in the list are all evaluated to `NULL`, the function returns `NULL`.

## Syntax

diagramsource

 case_value_expression  

````
case_value_expression ::=
    'CASE' ( 'WHEN' search_condition 'THEN' value_expression )+ ( 'ELSE' value_expression )? 'END'
  | 'coalesce' '(' value_expression ( ',' value_expression )+ ')'

````

## Parameters

### `search_condition`

A search condition that evaluates to a boolean type value.

- If the value is `TRUE`, the corresponding `value_expression` is returned. 
- If the value is `FALSE` or `UNKNOWN`, the next `search_condition` is evaluated.

### `value_expression`

A value expression that evaluates to a value. For more information, see [Value expressions](https://dev-docs.nebula-graph.io/v5.3.2/database-gql-reference/fe/#value_expressions).

## Examples

### Compare a list of values

````
'''
FOR a IN LIST[1,2,3,4,5,6,7,8,9,10]  
RETURN 
CASE 
WHEN a<4 THEN false 
WHEN a>5 THEN true 
END AS case_value
'''

````

Return:

````
+------------+
| case_value |
+------------+
| false      |
| false      |
| false      |
| null       |
| null       |
| true       |
| true       |
| true       |
| true       |
| true       |
+------------+

````

### Use `coalesce()` to return the first non-null value

````
RETURN coalesce(NULL, 1, 2, 3, 4, 5) AS a

````

Return:

````
+---+
| a |
+---+
| 1 |
+---+

````