create barcode using vb.net The ALTER TABLE and RENAME Commands in Java

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7.3 The ALTER TABLE and RENAME Commands
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Sometimes, it is necessary to change the structure of existing tables. For example, you may find that the maximum width of a certain column is defined too low, you might want to add an extra column to an existing table, or you may need to modify a constraint. In these situations, you can use the ALTER TABLE command. Figure 7-3 shows the syntax diagram for this command.
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DATA DEFINITION, PART II
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Figure 7-3. An ALTER TABLE command syntax diagram
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Note The ALTER TABLE command is much more complicated and extended than Figure 7-3 suggests. See Oracle SQL Reference for more details.
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You can add columns or constraint definitions to an existing table with the ADD option. The MODIFY option allows you to change definitions of existing columns. For example, you can widen a column, allow null values with NULL, or prohibit null values with NOT NULL. You can drop columns from tables with the DROP COLUMN option. You can also set columns to unused with the ALTER TABLE ... SET UNUSED command, and physically remove them from the database later with the ALTER TABLE ... DROP UNUSED COLUMNS command. This may be useful when you want to drop multiple columns in a single scan (accessing the rows only once). The RENAME COLUMN option allows you to change the name of a column.
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Caution You should be careful with the destructive DROP COLUMN option. Some database applications may depend on the existence of the column you are dropping.
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DATA DEFINITION, PART II
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With the constraint manipulation option, you can remove, enable, or disable constraints. Figure 74 shows the syntax details of this ALTER TABLE command option. For more details about constraint handling, see the next section.
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Figure 7-4. ALTER TABLE constraint manipulation syntax Just like the CREATE TABLE command, the ALTER TABLE command also allows you to influence various physical table storage attributes. In general, you can apply any structure change to existing tables, even when they contain rows. However, there are some exceptions. For example, for obvious reasons you cannot add a NOT NULL column to a nonempty table, unless you immediately specify a DEFAULT value in the same ALTER TABLE command. Listing 7-2 shows an example. Listing 7-2. ALTER TABLE Command Example SQL> alter table registrations 2 add (entered_by number(4) default 7839 not null); Table altered. SQL> alter table registrations 2 drop column entered_by; Table altered. SQL>
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Note The ALTER TABLE statement is probably the best illustration of the power of the relational model. Think about this: you can change a table definition while the table contains data and applications are running.
The RENAME command is rather straightforward. It allows you to change the name of a table or view (views are discussed in 10). Figure 7-5 shows the syntax diagram for the RENAME command.
DATA DEFINITION, PART II
Figure 7-5. RENAME command syntax diagram
7.4 Constraints
As you saw in the previous sections, you can specify constraint definitions in the CREATE TABLE and ALTER TABLE commands. As noted earlier in the description of the CREATE TABLE command, you can treat constraints as independent table components (for example, at the end of your CREATE TABLE command after all column definitions) or as part of a column definition. A common terminology to distinguish these two ways to specify constraints is out-of-line versus inline constraints. For each constraint definition, you can optionally specify a constraint name. It is highly recommended that you do so for all your constraint definitions. If you don t specify a constraint name yourself, the Oracle DBMS generates a far from informative name for you: SYS_Cnnnnn, where nnnnn is an arbitrary sequence number. Once constraints are created, you need their names to manipulate (enable, disable, or drop) them. Moreover, constraint names show up in constraint violation error messages. Therefore, well-chosen constraint names make error messages more informative. See Listing 7-3 later in this section for an example, showing a foreign key constraint violation.
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