How to Create Persistent Relations Using ALTER TABLE SQL

• Microsoft Visual FoxPro for Windows, version 3.0

SUMMARY

To create a persistent relation after tables are created, you can use the ALTER TABLE SQL command and its associated clauses.

MORE INFORMATION

A persistent relation must meet these requirements:

• The tables participating in the persistent relation must reside in a .DBC file because a persistent relation is considered a property of the DBC. Each persistent relation is a record in the DBC.
• The parent table must have primary-key type of index.

   CREATE DATABASE pr_dbc
   CREATE TABLE pr_Parent (cid c(3), cfld2 c(3))
   CREATE TABLE pr_Child (cid c(3), cfld2 c(3), cfld3 c(3))
   MODIFY DATABASE

Example One: Using the ALTER TABLE SQL Command to Create a Persistent Table

   ALTER TABLE pr_Parent ADD PRIMARY KEY cid TAG p_cid
   ALTER TABLE pr_Child ALTER COLUMN cid c(3) ;
      REFERENCES pr_Parent TAG p_cid

The first line of code adds a primary key index to the pr_Parent table with the ADD PRIMARY KEY clause. The first portion of ADD PRIMARY KEY identifies the field (cid) or expression that is to be indexed. The second portion, the TAG clause, allows a name to be specified for the index. This clause is required if the ADD PRIMARY KEY <expression> is anything other than a column name. If the ADD PRIMARY KEY <expression> is a column name and the TAG clause is not used, Visual FoxPro, by default, uses the column name as a TAG name.

The second line of code adds an ordinary index to the pr_child table on the column (cid) specified in the ALTER COLUMN clause. The REFERENCE clause identifies the parent table (pr_Parent) of the relation. The TAG portion of the REFERENCES clause is optional if there already is a primary key existing in the specified parent table.

If the primary key had is not predefined in the parent table and the TAG clause is not used, Visual FoxPro generates this error message:

   No Primary Key.

Example Two: Using the ALTER TABLE SQL Command to Create a Persistent Table

   ALTER TABLE pr_Parent ALTER COLUMN cid c(3) PRIMARY KEY
   ALTER TABLE pr_Child ALTER COLUMN cid c(3) ;
      ADD FOREIGN KEY cid TAG cc_id REFERENCES ar_parent TAG cid

The first line of code adds a primary key index based on the cid column with the default name of cid. This method does not provide a method of naming the TAG, to name the tag see the first line of example one.

The second line establishes the persistent relation between pr_Child and pr_Parent. It creates an index TAG on the column (cid) specified in the ADD FOREIGN KEY clause of the command. The TAG clause, is required here and can be used to give the TAG a more descriptive name.

The REFERENCES clause specifies the table that is to be the parent. The TAG portion of the REFERENCES clause is optional if there is already a primary key existing in the specified parent table. If the primary key is not predefined in the parent table and the TAG clause is not used, Visual FoxPro generates this error message:

   No Primary Key.

   ALTER TABLE pr_Child ADD FOREIGN KEY cid TAG cc_id ;
      REFERENCES ar_parent