ABAP News for Release 7.51 – Common Table Expressions (CTE) in Open SQL

When I mentioned CTEs one year ago, the reaction was that I’m “teasing“. But well, here we are. The Open SQL boys are really a hard-working bunch.

The wish was to have a SELECT … FROM subquery in Open SQL. That’s one thing, you can use CTEs for:

    WITH
      +conns AS (
        SELECT carrname, connid, cityfrom, cityto
              FROM spfli
                JOIN scarr ON spfli~carrid = scarr~carrid
              WHERE spfli~carrid = @carrid ),
      +cnts AS (
        SELECT COUNT(*) AS cnt
               FROM +conns )
      SELECT *
             FROM +cnts
               CROSS JOIN +conns
             ORDER BY carrname, connid
             INTO CORRESPONDING FIELDS of TABLE @itab.

The new SQL keyword WITH introduces the definition of one or more CTEs. Each CTE is a kind of subquery that produces a tabular result that can be used as a data source in all subsequent queries of the same WITH statement. In the example, a general table expression +conns (where the + is a mandatory prefix of the name) creates a table of flight connections. A further expression, +cnts, determines the number of entries in the result set of +conns. The closing mainquery combines the result sets of both general table expressions using a cross join (also new in ABAP 7.51).

WITH 
  +cities AS ( 
    SELECT cityfrom AS city 
           FROM spfli 
           WHERE carrid = @carrid 
    UNION DISTINCT 
    SELECT cityto AS city 
           FROM spfli 
           WHERE carrid = @carrid ) 
  SELECT * 
         FROM sgeocity 
         WHERE city IN ( SELECT city 
                                FROM +cities ) 
         INTO TABLE @DATA(result). 

In this example, there is one CTE +cities defining a subquery including a UNION. The result set of the CTE is evaluated as the data source of the subquery of the WHERE condition of the main query of the WITH statement.

The general CTE syntax is as follows:

WITH 
  +cte1[( name1, name2, ... )] AS ( SELECT subquery_clauses ... ), 
  +cte2[( name1, name2, ... )] AS ( SELECT subquery_clauses ... ), 
   ... 
  SELECT mainquery_clauses 
         ...
         INTO ...
       ... 
[ENDWITH]. 

Each WITH statement must be completed by a mainquery that uses at least one of its CTEs as a data source and each CTE must be used in at least one other subsequent query. Up to now a CTE cannot use itself as a data source. WITH can be used as a standalone statement or after the statement OPEN CURSOR. 

The ENDWITH is an ABAP specialty that fulfills the same task as ENDSELECT. If you don’t write in to an internal table behind INTO of the main query, you open a WITH loop that must be closed with ENDWITH.

With  ( name1, name2, … ) you can redefine the column names of the tabular result sets of a CTE, e.g.

WITH 
  +connections AS ( 
    SELECT spfli~carrid, carrname, connid, cityfrom, cityto 
           FROM spfli 
           INNER JOIN scarr 
             ON scarr~carrid = spfli~carrid 
           WHERE spfli~carrid BETWEEN @from_id AND @to_id ), 
  +sum_seats AS ( 
    SELECT carrid, connid, SUM( seatsocc ) AS sum_seats 
           FROM sflight 
           WHERE carrid BETWEEN @from_id AND @to_id 
           GROUP BY carrid, connid ), 
  +result( name, connection, departure, arrival, occupied ) AS ( 
    SELECT carrname, c~connid, cityfrom, cityto, sum_seats 
           FROM +connections AS c 
             INNER JOIN +sum_seats AS s 
               ON c~carrid = s~carrid AND 
                  c~connid = s~connid ) 
  SELECT * 
         FROM +result 
         ORDER BY name, connection 
         INTO TABLE @DATA(result).

The result sets of both general table expressions +connections and +sum_seats are merged in the subquery of the general table expression +result in a join expression. An explicit name list assigns names to the resulting columns. These names are used in the mainquery to sort the results.

With the introduction of WITH, the standalone statement SELECT suddenly lost a bit of its peculiarity. In fact it can be seen as a special case of a mainquery without WITH. Therefore the terminology of queries was generalized in Open SQL: Each language element SELECT implements a query and we distinguish between mainqueries and subqueries.

• A mainquery is either a standalone SELECT statement, the final query of a WITH statement, or the final query of an OPEN CURSOR statement.
• A subquery is possible in a WHERE condition, after WITH for the definition of a CTE, or as a data source of an INSERT statement.

All queries can be combined with UNION and the most clauses are common for all queries except the ABAP specific INTO that can be used for the mainqueries of a standalone WITH and a standalone SELECT and behind a FETCH only.