Similar to rows, you can define columns either when you define a table, or dynamically. Unlike rows, however, you can give names to columns so that you may refer to them more easily. You can also refer to columns by their position in the table. Column position is an integer or integer expression with a value from 1 to the number of columns. You can re-order columns at runtime; naturally, however, this changes their position indicator. Column names are not affected by changes in position.

The rules for legal column names are as follows:

•Each column name must be unique within its table.

•You can use the same column name in different tables.

•Do not give a column the same name as another element (variable, structure, etc.) in your program.

These rules facilitate the use of column names and of variables for column positions or names without requiring special syntax. They also allow the flexibility needed to reference columns and tables that are created dynamically.