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A database owner explicitly issues a checkpoint command to perform a checkpoint in that database. The log is getting full (more than 70 percent of capacity) and the database option trunc. log on chkpt. is set. A checkpoint is triggered to truncate the transaction log and free up space.

Part I:

4:

A column in one table whose value matches the primary key in some other table is called a foreign key In Figure 4-9, the REP_OFFICE column is a foreign key for the OFFICES table Although REP_OFFICE is a column in the SALESREPS table, the values that this column contains are office numbers They match values in the OFFICE column, which is the primary key for the OFFICES table Together, a primary key and a foreign key create a parent/child relationship between the tables that contain them, just like the parent/child relationships in a hierarchical database Just as a combination of columns can serve as the primary key of a table, a foreign key can also be a combination of columns In fact, the foreign key will always be a compound (multicolumn) key when it references a table with a compound primary key.

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KEY constraint is very similar to that of defining a UNIQUE constraint. If you want to add a PRIMARY KEY constraint to a column definition, use the following syntax: <column name> { <data type> | <domain> } PRIMARY KEY If you want to add a PRIMARY KEY constraint as a table constraint, you must add it as a table element in the table definition, as shown in the following syntax: [ CONSTRAINT <constraint name> ] PRIMARY KEY ( <column name> [ {, <column name> } . . . ] ) As with the UNIQUE constraint, you can use a column constraint to define a primary key if you re including only one column in the definition. For example, if you were to define a PRIMARY KEY constraint for the table shown in Figure 4-4, you would use the following SQL statement:

Obviously, the number of columns and the data types of the columns in the foreign key and the primary key must be identical to one another A table can contain more than one foreign key if it is related to more than one other table Figure 4-10 shows the three foreign keys in the ORDERS table of the sample database: The CUST column is a foreign key for the CUSTOMERS table, relating each order to the customer who placed it The REP column is a foreign key for the SALESREPS table, relating each order to the salesperson who took it The MFR and PRODUCT columns together are a composite foreign key for the PRODUCTS table, relating each order to the product being ordered The multiple parent/child relationships created by the three foreign keys in the ORDERS table may seem familiar to you, and they should.

They are precisely the same relationships as those in the network database of Figure 4-4 As the example shows, the relational data model has all of the power of the network model to express complex relationships..

CREATE TABLE CD_ARTISTS ( ARTIST_ID INT PRIMARY KEY, ARTIST_NAME VARCHAR(60), AGENCY_ID INT );

FIGURE 4-10

4:

If you want to apply the constraint to multiple columns or you simply want to keep it as a separate definition then you must use a table constraint:

Foreign keys are a fundamental part of the relational model because they create relationships among tables in the database. As with primary keys, foreign key support was missing from early relational database management systems. They were added to DB2 Version 2, were subsequently added to the ANSI/ISO standard, and now appear in all of the major commercial products.

CREATE TABLE CD_ARTISTS ( ARTIST_ID INT, ARTIST_NAME VARCHAR(60), AGENCY_ID INT, CONSTRAINT PK_ARTIST_ID PRIMARY KEY ( ARTIST_ID, ARTIST_NAME ) );

Codd s 12 Rules for Relational Databases*

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