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SQL2 CAST expression syntax diagram
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Subqueries and Query Expressions
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the sample database) and HIRE_DATE (a date in the sample database) into character strings for the returned query results:
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SELECT NAME, CAST REP_OFFICE AS VARCHAR, HIRE_DATE AS VARCHAR FROM SALESREPS
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The CAST expression can generally appear anywhere that a scalar-valued expression can appear within a SQL statement. In this example, it s used in the WHERE clause to convert a character string customer number into an integer, so that it can be compared with the data in the database:
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SELECT PRODUCT, QTY, AMOUNT FROM ORDERS WHERE CUST = CAST '2107' AS INTEGER
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Instead of specifying a data type in the CAST expression, you can specify a SQL2 domain. Domains are specific collections of legal data values that can be defined in the database under the SQL2 standard. They are fully described in 11 because of the role they play in SQL data integrity. Note that you can also generate a NULL value of the appropriate data type for use in SQL expressions using the CAST expression. The most common uses for the CAST expression are: I To convert data from within a database table where the column is defined with the wrong data type. For example, when a column is defined as a character string, but you know it actually contains numbers (that is, strings of digits) or dates (strings that can be interpreted as a month/day/year). I To convert data from data types supported by the DBMS that are not supported by a host programming language. For example, most host programming languages do not have explicit date and time data types and require that date/time values be converted into character strings for handling by a program. I To eliminate differences between data types in two different tables. For example, if an order date is stored in one table as DATE data, but a product availability date is stored in a different table as a character string, you can still compare the columns from the two tables by CASTing one of the columns into the data type of the other. Similarly, if you want to combine data from two different tables with a UNION operation, their columns must have identical data types. You can achieve this by CASTing the columns of one of the tables.
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The CASE Expression (SQL2)
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The SQL2 CASE expression provides for limited decision making within SQL expressions. Its basic structure, shown in Figure 9-10, is similar to the IF THEN ELSE statement found in many programming languages. When the DBMS encounters a CASE expression, it evaluates the first search condition, and if it is TRUE, then the value of the CASE expression
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Figure 9-10.
SQL2 CASE expression syntax diagram
is the value of the first result expression. If the result of the first search condition is not TRUE, the DBMS proceeds to the second search condition and checks whether it is TRUE. If so, the value of the CASE expression is the value of the second result expression, and so on. Here is a simple example of the use of the CASE expression. Suppose you want to do an A/B/C analysis of the customers from the sample database according to their credit limits. The A customers are the ones with credit limits over $60,000, the B customers are those with limits over $30,000 and the C customers are the others. Using SQL1, you would have to retrieve customer names and credit limits from the database and then rely on an application program to look at the credit limit values and assign an A, B, or C rating. Using a SQL2 CASE expression, you can have the DBMS do the work for you:
SELECT COMPANY, CASE WHEN CREDIT_LIMIT > 60000 THEN 'A' WHEN CREDIT_LIMIT > 30000 THEN 'B' ELSE 'C' FROM CUSTOMERS
For each row of query results, the DBMS evaluates the CASE expression by first comparing the credit limit to $60,000, and if the comparison is TRUE, returns an A in the second column of query results. If that comparison fails, the comparison to $30,000 is made and returns a B if this second comparison is TRUE. Otherwise, the third column of query results will return a C. This is a very simple example of a CASE expression. The results of the CASE expression are all literals here, but in general, they can be any SQL expression. Similarly, there is no requirement that the tests in each WHEN clause are similar, as they are here. The CASE expression can also appear in other clauses of a query. Here is an example of a query where it s useful in the WHERE clause. Suppose you want to find the total of the salesperson s sales, by office. If a salesperson is not yet assigned to an office, that person should be included in the total for his or her manager s office. Here is a query that generates the appropriate office groupings:
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