# Query using UDF query syntax
<a name="udf-query-syntax"></a>

The `USING EXTERNAL FUNCTION` clause specifies a UDF or multiple UDFs that can be referenced by a subsequent `SELECT` statement in the query. You need the method name for the UDF and the name of the Lambda function that hosts the UDF. In place of the Lambda function name, you can use the Lambda ARN. In cross-account scenarios, the Lambda ARN is required.

## Synopsis
<a name="udf-synopsis"></a>

```
USING EXTERNAL FUNCTION {{UDF_name}}({{variable1}} {{data_type}}[, {{variable2}} {{data_type}}][,...])
RETURNS {{data_type}}
LAMBDA '{{lambda_function_name_or_ARN}}'
[, EXTERNAL FUNCTION {{UDF_name2}}({{variable1}} {{data_type}}[, {{variable2}} {{data_type}}][,...]) 
RETURNS {{data_type}} 
LAMBDA '{{lambda_function_name_or_ARN}}'[,...]]
SELECT  [...] {{UDF_name}}({{expression}}) [, {{UDF_name2}}({{expression}})] [...]
```

## Parameters
<a name="udf-parameters"></a>

**USING EXTERNAL FUNCTION {{UDF\_name}}({{variable1}} {{data\_type}}[, {{variable2}} {{data\_type}}][,...])**  
{{UDF\_name}} specifies the name of the UDF, which must correspond to a Java method within the referenced Lambda function. Each {{variable data\_type}} specifies a named variable and its corresponding data type that the UDF accepts as input. The {{data\_type}} must be one of the supported Athena data types listed in the following table and map to the corresponding Java data type.      
[See the AWS documentation website for more details](http://docs.aws.amazon.com/athena/latest/ug/udf-query-syntax.html)

**RETURNS {{data\_type}}**  
`data_type` specifies the SQL data type that the UDF returns as output. Athena data types listed in the table above are supported. For the `DECIMAL` data type, use the syntax `RETURNS DECIMAL({{precision}}, {{scale}})` where {{precision}} and {{scale}} are integers.

**LAMBDA '{{lambda\_function}}'**  
{{lambda\_function}} specifies the name of the Lambda function to be invoked when running the UDF.

**SELECT [...] {{UDF\_name}}({{expression}}) [...]**  
The `SELECT` query that passes values to the UDF and returns a result. {{UDF\_name}} specifies the UDF to use, followed by an {{expression}} that is evaluated to pass values. Values that are passed and returned must match the corresponding data types specified for the UDF in the `USING EXTERNAL FUNCTION` clause.

### Examples
<a name="udf-examples"></a>

For example queries based on the [AthenaUDFHandler.java](https://github.com/awslabs/aws-athena-query-federation/blob/master/athena-udfs/src/main/java/com/amazonaws/athena/connectors/udfs/AthenaUDFHandler.java) code on GitHub, see the GitHub [Amazon Athena UDF connector](https://github.com/awslabs/aws-athena-query-federation/tree/master/athena-udfs) page.