Db2 for i - 动态 FROM 子句

Question

Environment: Db2 for i, version 7.3

Library/table structure:

CORPORATE/TENANTS
LIB01/INVOICE
LIB02/INVOICE
LIB03/INVOICE
…
LIBxx/INVOICE

The CORPORATE/TENANTS table contains a list of libraries where information about each tenant is stored. It has this structure and data:

CREATE OR REPLACE TABLE TENANTS (
ID     BIGINT GENERATED ALWAYS AS IDENTITY (START WITH 1),
TENANT CHAR(10) NOT NULL,
PRIMARY KEY(ID)
) RCDFMT TENANTSR;

RUNSQLSTM SRCFILE(HILLB/QDDLSRC) SRCMBR(TENANTS) DFTRDBCOL(CORPORATE)

+--+------+
|ID|TENANT|
+--+------+
| 1|LIB01 |
| 2|LIB02 |
|..|......|
|99|LIB99 |
+--+------+

The LIBxx/INVOICE tables are all identical to each other and have this structure:

CREATE OR REPLACE TABLE INVOICE (
ID     BIGINT GENERATED ALWAYS AS IDENTITY (START WITH 1),
PAYDAT INTEGER(6,0) NOT NULL,
AMOUNT DECIMAL(15,2) NOT NULL DEFAULT 0,
PRIMARY KEY(ID)
) RCDFMT INVOICER;

+--+------+------+
|ID|PAYDAT|AMOUNT|
+--+------+------+
| 1|180701|100.00|
| 2|180801| 35.00|
|..|......|......|
+--+------+------+

I want to generate a list of invoice amounts for all tenants for a given date:

180701 LIB01 100.00
180701 LIB02 140.00
180701 LIB03  74.00
…

Conceptually what I want to do is this (yes, I know this is invalid SQL):

SELECT PAYDAT, TENANT, AMOUNT
FROM $X.INVOICE
WHERE PAYDAT = 180701;

I want to pull data from the INVOICE table for each TENANT but I know the FROM clause cannot be dynamic like this. I'm sure this kind of query has a name but I don't know what it is so I'm unable to effectively use a search engine to find what I need.

This would be trivial to solve with an RPGLE program but I need a pure SQL solution.

Please note - the LIBxx values CANNOT be hardcoded in any way. These values can change at any time.

Answer 1

To do what you want, you can use a stored procedure with an EXECUTE IMMEDIATE in a loop to build a result set. Something like this:

Note: this is not a complete cut and paste solution, but you can modify it to do what you want.

CREATE OR REPLACE PROCEDURE GETINVOICEAMOUNTS ( ) 
  DYNAMIC RESULT SETS 1 
  LANGUAGE SQL 
  NOT DETERMINISTIC 
  MODIFIES SQL DATA 
  CALLED ON NULL INPUT 
  SET OPTION COMMIT = *NONE
BEGIN
  DECLARE STMT VARCHAR(1024);
  DECLARE RECORD_FOUND INTEGER DEFAULT 1;
  DECLARE LIBRARY CHAR(10);
  DECLARE C1 CURSOR FOR
    SELECT TENANT FROM CORPORATE/TENANT;
  DECLARE C2 CURSOR WITH RETURN FOR 
    SELECT * FROM SESSION.TMP ; 
  DECLARE CONTINUE HANDLER FOR NOT FOUND
    SET RECORD_FOUND = 0;
  DECLARE GLOBAL TEMPORARY TABLE TMP 
    (PAYDAT INTEGER(6,0),
     TENANT CHAR(10),
     AMOUNT DECIMAL(15,2))
    WITH REPLACE;
  OPEN C1;
  LOOP
    FETCH C1 INTO LIBRARY;
    IF RECORD_FOUND = 0;
      LEAVE LOOP;
    END IF;        
    SET STMT = 'INSERT INTO SESSION.TMP SELECT PAYDAT, LIBRARY, AMOUNT FROM ' || RTRIM(LIBRARY) || '.INVOICE WHERE PAYDAT = 180701';
    EXECUTE IMMEDIATE STMT
  END LOOP;
  CLOSE C1;
  OPEN C2; 
END; 

I gave you more than I planned to. But, one specific modification you will invariably need is to parameterize the date that you want to retrieve.

This is how it works: A global temporary table named TMP is used to collect the records to be returned in a result set. Once all the records are collected, a cursor is opened over TMP and the procedure ends. This causes the values collected in TMP to be returned as a result set.

To collect the values the CORPORATE/TENANT file is read, and the column TENANT is retrieved into the variable LIBRARY . For each record a statement is built that concatenates LIBRARY into an INSERT statement. This statement is executed which loads the record into TMP . I am using EXECUTE IMMEDIATE because I cannot use a parameter marker to replace the table reference in the INSERT statement, so a prepared statement is just extra work.

Answer 2

You could use UNION ALL :

SELECT sub.PAYDAT, t.TENANT, sub.AMOUNT
FROM (SELECT * FROM LIB01.INVOICE
      UNION ALL
      SELECT * FROM LIB02/INVOICE
      ...
      SELECT * FROM LIB0n/INVOICE) sub
JOIN TENANTS t
  ON sub.id = t.id
WHERE SUB.PAYDAT = 180701;

It is an SELECT * FROM sales + @yymm template.

---

EDIT:

More secure way is to create a view:

CREATE VIEW combined_invoice
AS
SELECT * FROM LIB01.INVOICE
UNION ALL
SELECT * FROM LIB02/INVOICE
...
SELECT * FROM LIB0n/INVOICE;

And query:

SELECT sub.PAYDAT, t.TENANT, sub.AMOUNT
FROM combined_invoice sub
JOIN TENANTS t
  ON sub.id = t.id
WHERE SUB.PAYDAT = 180701;

Of course view should be altered after adding/removing tables.