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Most programming languages do not provide SQL-style support for unknown or missing values. A variable in COBOL, C, or FORTRAN, for example, always has a value. There is no concept of the value being NULL or missing. This causes a problem when you want to store NULL values in the database or retrieve NULL values from the database using programmatic SQL. Embedded SQL solves this problem by allowing each host variable to have a companion host indicator variable. In an embedded SQL statement, the host variable and the indicator variable together specify a single SQL-style value, as follows: An indicator value of zero means that the host variable contains a valid value and that this value is to be used. A negative indicator value means that the host variable should be assumed to have a NULL value; the actual value of the host variable is irrelevant and should be disregarded. A positive indicator value means that the host variable contains a valid value, which may have been rounded off or truncated. This situation occurs only when data is retrieved from the database and is described later in the section Retrieving NULL Values. When you specify a host variable in an embedded SQL statement, you can follow it immediately with the name of the corresponding indicator variable. Both variable names are preceded by a colon. Here is an embedded UPDATE statement that uses the host variable amount with the companion indicator variable amount_ind:

- 19 -

18:

exec sql update salesreps set quota = :amount :amount_ind, sales = :amount2 where quota < 20000.00;

Dynamic SQL *

If amount_ind has a nonnegative value when the UPDATE statement is executed, the DBMS treats the statement as if it read

user for a table name and two column names, and constructs an UPDATE statement for the table that looks like this:

exec sql update salesreps set quota = :amount, sales = :amount2 where quota < 20000.00;

If amount_ind has a negative value when the UPDATE statement is executed, the DBMS treats the statement as if it read

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The user s input thus determines the table to be updated, the column to be updated, and the search condition to be used. The search comparison value and the updated data value are specified as parameters, to be supplied later when the UPDATE statement is actually executed. After building the UPDATE statement text in its buffer, the program asks the DBMS to compile it with the PREPARE statement. The program then enters a loop, prompting the user to enter pairs of parameter values to perform a sequence of table updates. This user dialog shows how you could use the program in Figure 18-4 to update the quotas for selected salespeople:

An important role of a database is to protect the stored data from access by unauthorized users. For example, suppose your assistant, named Mary, was not previously authorized to insert data about new customers into the database. This SQL statement grants her that permission: GRANT INSERT ON CUSTOMERS TO MARY Privilege granted. Similarly, the following SQL statement gives Mary permission to update data about customers and to retrieve customer data with the SELECT statement: GRANT UPDATE, SELECT ON CUSTOMERS TO MARY Privilege granted. If Mary is no longer allowed to add new customers to the database, this REVOKE statement will disallow it: REVOKE INSERT ON CUSTOMERS FROM MARY Privilege revoked. Similarly, this REVOKE statement will revoke all of Mary's privileges to access customer data in any way: REVOKE ALL ON CUSTOMERS FROM MARY Privilege revoked.

exec sql update salesreps set quota = NULL, sales = :amount2 where quota < 20000.00;

Enter name of table to be updated: staff Enter name of column to be searched: empl_num Enter name of column to be updated: quota Enter search value for empl_num: 106 Enter new value for quota: 150000.00 Another (y/n) y Enter search value for empl_num: 102 Enter new value for quota: 225000.00 Another (y/n) y Enter search value for empl_num: 107 Enter new value for quota: 215000.00 Another (y/n) n Updates complete.

A host variable/indicator variable pair can appear in the assignment clause of an embedded UPDATE statement (as shown here) or in the values clause of an embedded INSERT statement. You cannot use an indicator variable in a search condition, so this embedded SQL statement is illegal:

This program is a good example of a situation where two-step dynamic execution is appropriate. The DBMS compiles the dynamic UPDATE statement only once but executes it three times, once for each set of parameter values entered by the user. If the program had been written using EXECUTE IMMEDIATE instead, the dynamic UPDATE statement would have been compiled three times and executed three times. Thus, the two-step dynamic execution of PREPARE and EXECUTE helps to eliminate some of the performance disadvantage of dynamic SQL. As mentioned earlier, this same two-step approach is used by all of the callable SQL APIs described in 19.

exec sql delete from salesreps where quota = :amount :amount_ind;

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