Oracle Unleashed- P2: When I first started using Oracle many years ago, it was possible to know the database and the tools available. With the rash of recent releases of different options for the database and the spate of new tools, only people who wear their underpants over their trousers will be able to know everything there is to know about the Oracle products. | Because integrity constraints are checked at the database level they are performed regardless of where the insert update delete statement originated whether it was an Oracle or a non-Oracle tool. Defining checks using these constraints is also quicker than performing the same checks using SQL. In addition the information provided by declaring constraints is used by the Oracle optimizer to make better decisions about how to run a statement against the table. The Oracle Forms product can also use constraints to automatically generate code in the front-end programs to provide an early warning to the user of any errors. The types of integrity constraints that you can set up on a table are NOT NULL PRIMARY KEY UNIQUE FOREIGN KEY CHECK and indexes. NOT NULL Constraints You set the NOT NULL constraint against a column to specify that the column must always have a value on every row it can never be null. By default all the columns in a table are nullable. For example using a NOT NULL constraint on an orders table you can specify that there must always be an order amount. PRIMARY KEY The PRIMARY KEY constraint defines a column or a set of columns that you can use to uniquely identify a single row. No two rows in the table can have the same values for the primary key columns. In addition the columns for a primary key constraint must always have a value in other words they are NOT NULL. If you add a constraint to a table after it has been created any columns that make up the PRIMARY KEY constraint are modified to NOT NULL. Only one PRIMARY KEY constraint can exist for any table. For example using a PRIMARY KEY constraint on an orders table you can specify that a table cannot have two records with the same order number. UNIQUE The UNIQUE constraint defines a secondary key for the table. This is a column or set of columns that you can use as another way of uniquely identifying a row. No two rows can have the same values for the UNIQUE key column or columns. Although it is not .
