INFO: Outer Join with Three or More Data Tables (100069)

SUMMARY

When two data tables are joined through a SELECT-SQL statement, only those records that have entries in both the parent and child databases are included in the resulting data table. If it is necessary to also include parent records that do not have corresponding child entries, an outer join is performed.

A SQL outer join creates one data table that includes parent records--both those with and those without corresponding child records--and fields from both the parent and child databases.

Using an outer join, you can create one final result set from three or more databases (where the second and subsequent databases are each children of the parent) that includes fields from all databases regardless of whether the parent has records in any one or more of the child databases. The procedure for creating this single result set is detailed below.

NOTE: Visual FoxPro 5.0 and later support the OUTER JOIN syntax, which makes the techniques described in this article unnecessary. For more information, please see the "References" section.

MORE INFORMATION

To create one result set from three databases, where one database is the parent and the two other databases are children of the same parent, do the following:

Using the CUSTOMER database located in the GOODIES\TUTORIAL subdirectory in the main FoxPro directory or in the SAMPLES\DATA directory in Visual FoxPro, create the following sample data set:
```
lcVersion = VERSION()

DO CASE
	CASE "Visual FoxPro 03" $ lcVersion OR "Visual FoxPro 05" $ lcVersion
	    SET DEFAULT TO HOME() + "SAMPLES\DATA"
	CASE "Visual FoxPro 06" $ lcVersion
		SET DEFAULT TO HOME(2) + "DATA"
	OTHERWISE
	    SET DEFAULT TO HOME() + "TUTORIAL"
ENDcase

USE customer
COPY TO child1 NEXT 50
SKIP -10
COPY TO child2 NEXT 50
					
```
The parent file will now have 100 children in CHILD1 and 50 different children in CHILD2. For several customer numbers, child records will exist both in CHILD1 and CHILD2.
Determine which fields are needed from the parent and each child.

Perform an outer join between the parent (CUSTOMER) and the first child (CHILD1). Store the results in a cursor so you don't have to eliminate temporary files later:

IF "Visual" $ VERSION()
	SELECT customer.cust_id, child1.company ;
		FROM customer, child1 ;
		WHERE customer.cust_id = child1.cust_id ;
	UNION ;
	SELECT customer.cust_id, "" FROM customer ;
		WHERE customer.cust_id NOT IN ;
			(SELECT child1.cust_id ;
				FROM child1 ;
				WHERE customer.cust_id = child1.cust_id) ;
		INTO CURSOR oj1
ELSE
	SELECT customer.cno, child1.company ;
		FROM customer, child1 ;
		WHERE customer.cno = child1.cno;
	UNION ;
	SELECT customer.cno, "" FROM customer ;
		WHERE customer.cno NOT IN ;
		(SELECT child1.cno ;
			FROM child1 ;
			WHERE customer.cno = child1.cno ) ;
		INTO CURSOR oj1
ENDif

NOTE: The placeholder for CHILD1.COMPANY is critical, since this field would otherwise be blank whenever there was not a match between the parent and child. This would result in a huge output data set, since an attempt would be made to join each blank CHILD1.COMPANY field with each CUSTOMER.CNO record. Use a placeholder for any field in the child database that was selected in the first half of the outer join command.

Perform an outer join of the parent and CHILD2:

IF "Visual" $ VERSION()
	SELECT customer.cust_id, child2.contact, child2.maxOrdAmt ;
		FROM customer, child2 ;
		WHERE customer.cust_id = child2.cust_id ;
	UNION ;
	SELECT customer.cust_id, "", NTOM(0.00) FROM customer ;
		WHERE customer.cust_id NOT IN ;
			(SELECT child2.cust_id FROM child2 ;
				WHERE customer.cust_id = child2.cust_id) ;
		INTO CURSOR oj2 
ELSE
	SELECT customer.cno, child2.contact, child2.lat ;
		FROM customer, child2 ;
		WHERE customer.cno = child2.cno ;
	UNION ;
	SELECT customer.cno, "", 0 FROM customer;
		WHERE customer.cno NOT IN ;
			(SELECT child2.cno ;
				FROM child2 ;
				WHERE customer.cno = child2.cno) ;
		INTO CURSOR oj2
ENDif

Repeat the outer join for each child. (NOTE: Capacity will be limited by the number of available file handles, open work areas, free disk space, and RAM.)

Perform a join of each of the temporary data sets that were created in steps 3 through 5:

IF "Visual" $ VERSION()
	SELECT oj1.cust_id, oj1.company, oj2.contact, oj2.maxOrdAmt ;
		FROM oj1, oj2 ;
		WHERE oj1.cust_id = oj2.cust_id ;
		INTO CURSOR ojFinal
ELSE
	SELECT oj1.cno, oj1.company, oj2.contact, oj2.lat ;
		FROM oj1, oj2 ;
		WHERE oj1.cno = oj2.cno ;
		INTO CURSOR ojFinal
ENDif

Use a BROWSE command to examine the results:
```
SELECT ojfinal
BROWSE
						
```
Examine the results. OJFINAL should contain the same number of records as exist in CUSTOMER.DBF. In FoxPro 2.x, all records should contain an entry in the CNO field. One hundred records should contain an entry in the COMPANY field. Fifty records should contain entries in the CONTACT and LAT fields. The COMPANY, CONTACT, and LAT fields should be blank in the majority of output records.

In Visual FoxPro, all records should contain an entry in the CUST_ID field. Fifty records should contain an entry in the COMPANY field. Fifty records should contain entries in the CONTACT and MAXORDAMT fields. Two or three records, depending on the version, should contain no records in COMPANY, CONTACT, and MAXORDAMT.

INFO: Outer Join with Three or More Data Tables (100069)

SUMMARY

MORE INFORMATION

REFERENCES