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The CUSTOMERS table has been replaced by two new tables, CUST_INFO and ACCOUNT_INFO, and is no longer needed.
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DROP TABLE CUSTOMERS
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Sam gives you permission to drop his table, named BIRTHDAYS.
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When the DROP TABLE statement removes a table from the database, its definition and all of its contents are lost. There is no way to recover the data, and you would have
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to use a new CREATE TABLE statement to recreate the table definition. Because of its serious consequences, you should use the DROP TABLE statement with care. The SQL2 standard requires that a DROP TABLE statement include either CASCADE or RESTRICT, which specifies the impact of dropping a table on other database objects (such as views, described in 14) that depend on the table. If CASCADE is specified, the DROP TABLE statement fails if other database objects reference the table. Most commercial DBMS products accept the DROP TABLE statement with no option specified.
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Changing a Table Definition (ALTER TABLE)
After a table has been in use for some time, users often discover that they want to store additional information about the entities represented in the table. In the sample database, for example, you might want to: I Add the name and phone number of a key contact person to each row of the CUSTOMERS table, as you begin to use it for contacting customers I Add a minimum inventory-level column to the PRODUCTS table, so the database can automatically alert you when stock of a particular product is low I Make the REGION column in the OFFICES table a foreign key for a newly created REGIONS table, whose primary key is the region name I Drop the foreign key definition linking the CUST column in the ORDERS table to the CUSTOMERS table, replacing it with two foreign key definitions linking the CUST column to the newly created CUST_INFO and ACCOUNT_INFO tables Each of these changes, and some others, can be handled with the ALTER TABLE statement, shown in Figure 13-4. As with the DROP TABLE statement, you will normally use the ALTER TABLE statement on one of your own tables. With proper permission, however, you can specify a qualified table name and alter the definition of another user s table. As shown in the figure, the ALTER TABLE statement can: I Add a column definition to a table I Drop a column from a table I Change the default value for a column I Add or drop a primary key for a table I Add or drop a new foreign key for a table I Add or drop a uniqueness constraint for a table I Add or drop a check constraint for a table The clauses in Figure 13-4 are specified in the SQL standard. Many DBMS brands lack support for some of these clauses or offer clauses unique to the DBMS, which alters other table characteristics. The SQL2 standard restricts each ALTER TABLE statement to a single table change. To add a column and define a new foreign key, for
13:
Creating a Database
Figure 13-4.
ALTER TABLE statement syntax diagram
example, requires two separate ALTER TABLE statements. Several DBMS brands relax this restriction and allow multiple action clauses in a single ALTER TABLE statement.
Adding a Column
The most common use of the ALTER TABLE statement is to add a column to an existing table. The column definition clause in the ALTER TABLE statement is just like the one in the CREATE TABLE statement, and it works the same way. The new column is added to the end of the column definitions for the table, and it appears as the rightmost column in subsequent queries. The DBMS normally assumes a NULL value for a newly added column in all existing rows of the table. If the column is declared to be NOT NULL with a default value, the DBMS instead assumes the default value. Note that you cannot simply declare the new column NOT NULL, because the DBMS would assume NULL values for the column in the existing rows, immediately violating the constraint! (When you add a new column, the DBMS doesn t actually go through all of the existing rows of the table adding a NULL or default value. Instead, it detects the fact that an existing row is too short for the new table definition when the row is retrieved, and extends it with a NULL or default value before displaying it or passing it to your program.) Some sample ALTER TABLE statements that add new columns are: Add a contact name and phone number to the CUSTOMERS table.
ALTER TABLE CUSTOMERS ADD CONTACT_NAME VARCHAR(30)
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