src/Query/FluentQuery.php
public function expression( $function, $column, $as ) {
		$this->select->expression( $function, $this->prepare_column( $column ), $as );

		$this->expressions[] = $as;

		return $this;
	}
public function expression( $function, $column, $as ) {
		$this->select->expression( $function, $this->prepare_column( $column ), $as );

		$this->expressions[] = $as;

		return $this;
	}
Select an expression, such as 'COUNT'.

@since 2.0

@param string $function
@param string $column
@param string $as

@return $this