Hi Everyone,
In relational theory, a join is the projection and restriction of the 
roduct.
The product, or Cartesian product, is the set of all possible combinations of 
the rows from the two tables.
The system first examines all possible combinations of the rows from two 
tables, then eliminates all the rows that do not meet the conditions of the 
projection (columns) and restriction (selection of rows).  The actual 
procedure that FoxPro follows is a bit more sophisticated, and varies from 
implementation to implementation.
With join queries, more so than with other types of SQL queries, you should 
test your queries on a sample of your data to make sure that you're getting 
the results you really want.  An insufficiently restricted join query can 
return results that need a good pruning before you can be sure that they are 
correct, or useful.  Also, you can bring down your LAN with the amount of 
.TMP file space and the large projection created!  I've done it, lads.
What makes a good join column?  Ideally, a join column is a key column for 
its table, either a primary key or a foreign key.  A primary key requires 
uniqueness of its data column or columns.  When a key is a composite, you 
must join on all the columns of the key.
Since the primary key logically connects to related foreign key columns in 
other tables, key columns are usually ideal candidate columns on which to 
construct a join.  Such a join is likely to be useful and logically 
appropriate, because you planned ahead for it.  Primary key-foreign key joins 
are based on the expectation that foreign keys will be kept consistent with 
their primary keys, in order to preserve the referential integrity of the 
entire database.
In FoxPro and VFP, columns being joined need not have the same name, though 
mine usually do.  I still like easy SET RELATION TO and UPDATE ON 
xpressions.
Although they need not be identical datatypes, the datatypes of join columns 
must be compatible -- types that Fox can convert from one type to another.  
For example, Fox can easily convert among any of the numeric type columns -- 
such as integer, decimal, currency, or float.  Datype-compatible join columns 
are, generally speaking, both character types, or both numeric types.
When you join tables, Fox compares the data in the specified fields and 
displays the results of the comparison as a table of the qualifying rows.  
The results show a row for each successful join.  Data from any of the tables 
is duplicated as needed.
Join query results replicate data from qualifying rows as needed in order to 
regularize the table display.  As with any query, the results of a join query 
display as a table without altering the database tables in any way.  The join 
operation simply permits the system to manipulate data from multiple tables 
as if it were contained in a single table.
* ---- I'm out of time.  More next time.  I'm leading to something here, so 
stay tuned.
David in Dallas.
--- Maximus/2 3.01
---------------