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Each table in a relational database should have a primary key. A primary key is a column, or set of columns, that uniquely identifies each row. No two rows in a table can have the same primary key value, and no column in a primary key can contain the NULL value.
Only base tables and global temporary tables can have primary keys. With declared temporary tables, you can create a unique index over a set of NOT NULL columns to mimic the semantics of a primary key.
It is recommended that you do not use approximate data types such as FLOAT and DOUBLE for primary keys or for columns with unique constraints. Approximate numeric data types are subject to rounding errors after arithmetic operations.
You can also specify whether to cluster the primary key index, using the CLUSTERED clause.
Primary key column order is determined by the order of the columns as specified in the primary key declaration of the CREATE TABLE (or ALTER TABLE) statement. You can also specify the sort order (ascending or descending) for each individual column. These sort order specifications are used by the database server when creating the primary key index.
The order of the columns in a primary key does not dictate the order of the columns in any referential constraints. You can specify a different column order, and different sort orders, with any foreign key declaration.
Managing primary keys (Sybase Central)
Managing primary keys (SQL)
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