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The CAST Expression
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The SQL standard has fairly restrictive rules about combining data of different types in expressions. It specifies that the DBMS shall automatically convert among very similar data types, such as 2-byte and 4-byte integers. However, if you try to compare numbers and character data, for example, the standard says that the DBMS should generate an error. The standard considers this an error condition even if the character string contains numeric data. You can, however, explicitly ask the DBMS to convert among data types using the CAST expression, which has the syntax shown in Figure 9-9.
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FIGURE 9-9 CAST expression syntax diagram
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The CAST expression tends to be of little importance when you are typing SQL statements directly into an interactive SQL interface. However, it can be critical when using SQL from within a programming language where data types don t match the data types supported by the SQL standard. For example, the CAST expression in the SELECT clause of this query converts the values for REP_OFFICE (integers in 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 CHAR), CAST (HIRE_DATE AS CHAR) FROM SALESREPS;
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Support for the CAST expression varies across SQL implementations. For example, Oracle requires that CHAR and VARCHAR data types used in a CAST expression include a length specification, while MySQL and DB2 UDB do not support the DATE data type in a CAST expression. 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:
SELECT PRODUCT, QTY, AMOUNT FROM ORDERS WHERE CUST = CAST ('2107' AS INTEGER);
Instead of specifying a data type in the CAST expression, you can specify a domain. Domains are specific collections of legal data values that can be defined in the database. 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 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 calendar date). 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. 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.
9:
Subqueries and Query Expressions
CASE
WHEN search-condition THEN
result-expression NULL ELSE result-expression NULL
FIGURE 9-10
CASE expression syntax diagram
The CASE Expression
The 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 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. Without the CASE expression, 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 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' END AS CREDIT_RATING FROM CUSTOMERS;
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