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In each case, the row is deleted to maintain the database as an accurate model of the real world. The smallest unit of data that can be deleted from a relational database is a single row.

The DELETE statement, shown in Figure 10-5, removes selected rows of data from a single table. The FROM clause specifies the target table containing the rows. The WHERE clause specifies which rows of the table are to be deleted. Suppose Henry Jacobsen, the new salesperson hired earlier in this chapter, has just decided to leave the company. The DELETE statement that removes his row from the SALESREPS table is shown next. Remove Henry Jacobsen from the database.

MAX_SOLD ------54

TABLE 16-9

This result, by itself, is not particularly helpful. It would be nice if your query results also included the name of the artist and the CD. However, SQL does not support a SELECT statement such as the following:

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Information Name of constraint (unique, primary key, or foreign key) described by this row Schema containing the constraint Table to which constraint applies Name of column in the constraint Position of column within the constraint (first column = 1)

The SYSCAT.KEYCOLUSE View (DB2)

SELECT ARTIST_NAME, CD_NAME, MAX(NUMBER_SOLD) FROM ARTIST_CDS;

16:

You can query the SYSCAT.COLUMNS table to find a table s primary key: List the columns that form the primary key of the PRODUCTS table.

Because set functions treat data as groups, you cannot specify the artist name and CD name without somehow grouping the data together. As it stands now, the MAX function treats the entire table as one group; however, neither the ARTIST_NAME values nor the CD_NAME values are grouped together in any way, so the SELECT clause becomes illogical. Whenever you include a set function in an SQL statement, then every argument in the SELECT list must either be a set function or be included in a group (using the GROUP BY clause described a little later in this topic). One way around this is to use a subquery in the WHERE clause to return the maximum value and then return the necessary information based on that value, as shown in the following example:

SELECT FROM WHERE AND ORDER COLNAME, KEYSEQ, TYPENAME, REMARKS SYSCAT.COLUMNS TABNAME = 'PRODUCTS' KEYSEQ > 0 BY KEYSEQ;

SELECT ARTIST_NAME, CD_NAME, NUMBER_SOLD FROM ARTIST_CDS WHERE NUMBER_SOLD = ( SELECT MAX(NUMBER_SOLD) FROM ARTIST_CDS );

The DB2 catalog support for primary and foreign keys is typical of that found in other major SQL products. The Oracle system ALL_CONSTRAINTS and USER_CONSTRAINTS views, for example, provide the same information as the DB2 SYSCAT.REFERENCES system table. Information about the specific columns that make up a foreign key or primary key appears in the Oracle ALL_CONS_COLUMNS and USER_CONS_COLUMNS system views, which are analogous to the DB2 SYSCAT.KEYCOLUSE system table. Microsoft SQL Server has a similar catalog structure, with foreign key information divided between the SYS. FOREIGN_KEYS and SYS.FOREIGN_KEY_COLUMNS catalog views. Informix Universal Server takes a similar approach to the DB2 catalog, but with the same types of differences previously illustrated in its table information and column information support. Each constraint defined within the database generates one row in the Informix SYSCONSTRAINTS system table, which defines the name of the constraint and its type (check constraint, primary key, referential, and so on). This system table also assigns an internal constraint-id number to identify the constraint within the catalog. The table to which the constraint applies is also identified by table-id (which serves as a foreign key to the SYSTABLES system table). Further information about the referential constraints (foreign keys) is contained in a SYSREFERENCES system table. Again in this table, the constraint, the primary key, and the parent table are identified by internal IDs that link the foreign key information to the SYSCONSTRAINTS and SYSTABLES system tables. The SYSREFERENCES table contains information about the delete rule and update rule that apply to the foreign key relationship and similar information.

The system catalog generally contains a table that identifies the users who are authorized to access the database. The DBMS may use this system table to validate the user name and password when a user first attempts to connect to the database. The table may also store other data about the user. SQL Server stores user information in its SYS.DATABASE_PRINCIPALS catalog view, shown in Table 16-10. Each row of this table describes a single user or user group in the SQL Server security scheme. Informix takes a similar approach, with a system table that is also

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