PRB: Using "!=" in a Join Statement Does Not Return Records

ID: Q176888

The information in this article applies to:

Microsoft Visual FoxPro for Windows, versions 5.0, 5.0a, 6.0

SYMPTOMS

When you specify a JOIN condition in a SQL Select statement using the ON clause, and you use the not equal sign (!=) with one empty table, the statement does not return any records.

STATUS

This behavior is by design.

MORE INFORMATION

ON Clause with !=

The following syntax does not return all the records from one table that do not have a matching record in the other table:

SELECT * FROM tbl1 INNER JOIN tbl2 ON tbl1.fld1 != tbl2.fld1

This creates a Cartesian product that excludes records that do match.

Where Clause with !=

To create a result set that includes all records from one table that do not exist in a second table, use the following syntax:

SELECT * FROM tbl1 WHERE tbl1.fld1 != tbl2.fld1

In conclusion, the following statements are not equivalent:


   SELECT * FROM tbl1 inner join tbl2 on tbl1.fld1 != tbl2.fld1
   SELECT * FROM tbl1 WHERE tbl1.fld1 != tbl2.fld1

Steps to Reproduce Behavior

Place the following code in a program file and run the code. Note that there are no records returned in the cursor.


   CREATE DATABASE MyTest
   CREATE TABLE tbl1 (fld1 c(10),fld2 c(20))
   CREATE TABLE tbl2 (fld1 c(10), fld2 c(20))
   INSERT INTO tbl1 VALUES('1','Rec1')
   INSERT INTO tbl1 VALUES ('2','Rec2')
   SELECT * INTO CURSOR MyCurse FROM tbl1 inner JOIN tbl2 ON ;
   tbl1.fld1 != tbl2.fld1

For a demonstration of the Cartesian product, run the following code with the database created in the code above open:


   INSERT INTO tbl2 VALUES ('1','tbl2Rec1')
   INSERT INTO tbl2 VALUES ('2','tbl2Rec2')
   SELECT * INTO CURSOR MyCurse FROM tbl1 inner JOIN tbl2 ON ;
   tbl1.fld1 != tbl2.fld1

REFERENCES

For additional information, please see the following article in the Microsoft Knowledge Base:

Q124589 How to Create a Left-Right Outer Join in FoxPro for Windows

Additional query words: SQL Select

Keywords : kbVFp kbVFp500 kbVFp500a kbVFp600 FxprgGeneral FxprgSql
Version : WINDOWS:5.0,5.0a,6.0
Platform : WINDOWS
Issue type : kbprb