private function sql_select() {

		$selects = array();

		// Always select the ID.
		$selects[] = 'u.ID as id';

		// Always add the subqueries for enrollment status.
		$selects[] = "( {$this->sql_subquery( 'meta_value' )} ) AS status";

		// All the possible fields.
		$fields = array(
			'completed'        => "( {$this->sql_subquery( 'updated_date', '_is_complete' )} ) AS completed",
			'date'             => "( {$this->sql_subquery( 'updated_date' )} ) AS `date`",
			'last_name'        => 'm_last.meta_value AS last_name',
			'first_name'       => 'm_first.meta_value AS first_name',
			'email'            => 'u.user_email AS email',
			'registered'       => 'u.user_registered AS registered',
			'overall_progress' => 'CAST( m_o_p.meta_value AS decimal( 5, 2 ) ) AS overall_progress',
			'overall_grade'    => 'CAST( m_o_g.meta_value AS decimal( 5, 2 ) ) AS overall_grade',
		);

		// Add the fields to the array of fields to select.
		foreach ( $fields as $key => $statement ) {
			if ( $this->requires_field( $key ) ) {
				$selects[] = $statement;
			}
		}

		$sql = implode( ', ', $selects );
		$sql = $this->sql_select_columns( $sql );

		if ( $this->get( 'suppress_filters' ) ) {
			return $sql;
		}

		/**
		 * Filters the query SELECT clause
		 *
		 * @since 3.13.0
		 *
		 * @param string             $sql           The SELECT clause of the query.
		 * @param LLMS_Student_Query $student_query Instance of LLMS_Student_Query.
		 */
		return apply_filters( 'llms_student_query_select', $sql, $this );

	}