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Host Variables and NULL Values
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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: I An indicator value of zero means that the host variable contains a valid value and that this value is to be used. I 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.
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SMALLINT INTEGER REAL DOUBLE PRECISION NUMERIC(p,s) DECIMAL(p,s) CHAR(n) VARCHAR(n) BIT(n) BIT VARYING(n) DATE TIME TIMESTAMP INTERVAL
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FORTRAN Type PL/I Type
INTEGER*2 FIXED BIN(15) FIXED BIN(31) BIN FLOAT(21) BIN FLOAT(53)
PIC S9 (9) COMP INTEGER*4 COMP-1 COMP-2 PIC S9 (p-s) V9(s) COMP-3 PIC X (n) Req. conv. PIC X (l) Req. conv.
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FIXED DEC(p,s) CHAR(n) CHAR(n) VAR BIT(n) BIT(n) VAR Req. conv.
CHARACTER*n Req. conv.
CHARACTER*L3 Req. conv. Req. conv.
char x[1]3 Req. conv.
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Notes: 1 Host language does not support packed decimal data; conversion to or from floating point data may cause truncation or round-off errors.
The SQL standard specifies a C string with a null terminator; older DBMS implementations returned a separate length value in a data structure. The length of the host character string (l) is the number of bits (n), divided by the bits-per-character for the host language (typically 8), rounded up. Host language does not support variable-length strings; most DBMS brands will convert to fixed-length strings. Host languages do not support native date/time data types; requires conversion to/from host language character string data types with text date, time, and interval representations.
PROGRAMMING WITH SQL
Table 17-1.
SQL Data Types
I 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.
SQL: The Complete Reference
. . . exec sql int char float char exec sql
begin declare section; hostvar1 = 106; *hostvar2 = "Joe Smith"; hostvar3 = 150000.00; *hostvar4 = "01-JUN-1990"; end declare section;
exec sql update salesreps set manager = :hostvar1 where empl_num = 102; exec sql update salesreps set name = :hostvar2 where empl_num = 102: exec sql update salesreps set quota = :hostvar3 where empl_num = 102; exec sql update salesreps set hire_date = :hostvar4 where empl_num = 102; . . .
Figure 17-18.
Host variables and data types
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:
exec sql update salesreps set quota = :amount :amount_ind, sales = :amount2 where quota < 20000.00;
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Embedded SQL
If amount_ind has a nonnegative value when the UPDATE statement is executed, the DBMS treats the statement as if it read:
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:
exec sql update salesreps set quota = NULL, sales = :amount2 where quota < 20000.00;
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:
exec sql delete from salesreps where quota = :amount :amount_ind;
This prohibition exists for the same reason that the NULL keyword is not allowed in the search condition it makes no sense to test whether QUOTA and NULL are equal, because the answer will always be NULL (unknown). Instead of using the indicator variable, you must use an explicit IS NULL test. This pair of embedded SQL statements accomplishes the intended task of the preceding illegal statement:
if (amount_ind < 0) { exec sql delete from salesreps where quota is null; } else { exec sql delete from salesreps where quota = :amount; }
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