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PART IV
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Part IV:
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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 may not actually go through all of the existing rows of the table adding a NULL or default value. Instead, some DBMS products detect the fact that an existing row is too short for the new table definition when the row is retrieved, and extend 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.
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ALTER TABLE CUSTOMERS ADD CONTACT_NAME VARCHAR(30); ALTER TABLE CUSTOMERS ADD CONTACT_PHONE CHAR(10);
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Add a minimum inventory-level column to the PRODUCTS table.
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ALTER TABLE PRODUCTS ADD MIN_QTY INTEGER NOT NULL DEFAULT 0;
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In the first example, the new columns will have NULL values for existing customers. In the second example, the MIN_QTY column will have the value zero (0) for existing products, which is appropriate. When the ALTER TABLE statement first appeared in SQL implementations, the only major structures within a table were the column definitions, and it was very clear what the ADD clause meant. Since then, tables have grown to include primary and foreign key definitions and constraints, and the ADD clauses for these types of objects specify which type of object is being added. For consistency with these other ADD/DROP clauses, the SQL standard includes the optional keyword COLUMN after the keyword ADD. With this addition, the preceding example becomes Add a minimum inventory-level column to the PRODUCTS table.
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ALTER TABLE PRODUCTS ADD COLUMN MIN_QTY INTEGER NOT NULL DEFAULT 0;
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Dropping a Column
The ALTER TABLE statement can be used to drop one or more columns from an existing table when they are no longer needed. Here is an example that drops the HIRE_DATE column from the SALESREPS table: Drop a column from the SALESREPS table.
ALTER TABLE SALESREPS DROP HIRE_DATE;
The SQL standard forces you to issue a separate ALTER TABLE statement if you want to drop several columns, but several of the major DBMS brands allow you to drop multiple columns with a single statement.
13:
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Note that dropping a column can pose the same kinds of data-integrity issues that were described in 11 for database update operations. For example, if you drop a column that is a primary key in some relationship, the foreign key columns that refer to the dropped column become invalid. A similar problem can arise if you drop a column that is referenced in a check constraint the column that provides the data value for checking the constraint is now gone. A similar problem is created in views that are defined based on the dropped column. The SQL standard deals with these issues the same way it handled the potential dataintegrity problems posed by DELETE and UPDATE statements with a drop rule (actually called a drop behavior in the standard) that operates just like the delete rules and update rules. You can specify one of two drop rules: RESTRICT If any other objects in the database (foreign keys, constraints, and so on) depend on the column to be dropped, the ALTER TABLE statement fails with an error and the column is not dropped. CASCADE Any other objects in the database (foreign keys, constraints, and so on) that depend on the column are also dropped as a cascaded effect of the ALTER TABLE statement. The CASCADE effect can cause quite dramatic changes in the database; therefore, use it with care. It s usually a better idea to use the RESTRICT mode (explicitly drop the dependent foreign keys and constraints, using the appropriate ALTER or DROP statements) before dropping the column. The other common use for the ALTER TABLE statement is to change or add primary key and foreign key definitions for a table. Many find this form of the ALTER TABLE statement is particularly useful. It can be used to inform the DBMS about intertable relationships that already exist in a database, but which have not been explicitly specified before. Unlike column definitions, primary key and foreign key definitions can be added and dropped from a table with the ALTER TABLE statement. The clauses that add primary key and foreign key definitions are exactly the same as those in the CREATE TABLE statement, and they work the same way. The clauses that drop a primary key or foreign key are straightforward, as shown in the following examples. Note that you can drop a foreign key only if the relationship that it creates was originally assigned a name. If the relationship was unnamed and the DBMS is not one of those that assigns default names to unnamed constraints, there is no way to specify it in the ALTER TABLE statement. In this case, you cannot drop the foreign key unless you drop and re-create the table, using the procedure described for dropping a column. Here is an example that adds a foreign key definition to an existing table: Make the REGION column in the OFFICES table a foreign key for the newly created REGIONS table, whose primary key is the region name.
ALTER TABLE ADD CONSTRAINT FOREIGN KEY REFERENCES OFFICES INREGION (REGION) REGIONS;
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