Rule Definition
The SELECT ... ENDSELECT works as a loop fetching single record for every loop pause. Basically, it works like a client cursor which will generate to much traffic on the network and communications between the application server and the database server. Having nested SELECT ... ENDSELECT is equivalent to have an SQL query inside a loops which number of iterations is based on the number of rows returned by the SQL query. Having a such a nested statement means having an SQL query inside a loop which number of iterations can be very high (dependent on the data returned from the database). This iterative pattern has proved to be dangerous for application performance and scalability. Database servers handle in a much better set-oriented pattern rather than pure iterative ones. Load data in internal tables instead.

Remediation
It is recommended either to modify the queries in order to use one single query using a set-oriented pattern rather than pure iterative one. Or to use SAP ABAP internal tables instead : "SELECT... INTO TABLE...+ LOOP AT Table" statement.

Violation Code Sample

SELECT * FROM vbak INTO wa_vbak
         WHERE vbeln IN so_vbeln.
  SELECT * FROM vbap INTO wa_vbap
         WHERE vbeln = wa_vbak-vbeln.
    SELECT SINGLE * FROM mara INTO wa_mara
         WHERE matnr EQ wa_vbap-matnr.
  ENDSELECT.
ENDSELECT.

Fixed Code Sample

Remediation example:

SELECT vbeln auart FROM vbak INTO TABLE ts_vbak
         WHERE vbeln IN so_vbeln.
         
IF lines( ts_vbak ) > 0
  SELECT vbeln posnr matnr kwmeng vrkme
         FROM vbap
         INTO TABLE ts_vbap
         FOR ALL ENTRIES IN ts_vbak
         WHERE vbeln = ts_vbak-vbeln.
         
  LOOP AT ts_vbak INTO wa_vbak.
    LOOP AT ts_vbap INTO wa_vbap
         WHERE vbeln EQ wa_vbak-vbeln.
      SELECT SINGLE * FROM mara INTO wa_mara
         WHERE matnr EQ wa_vbap-matnr.
    ENDLOOP.
  ENDLOOP.
ENDIF.

MIT ABAP Check List http://web.mit.edu/ist/org/admincomputing/dev/abap_review_check_list.htm