| PostgreSQL 8.2.23 Documentation | ||||
|---|---|---|---|---|
| Prev | Fast Backward | Chapter 31. ECPG - Embedded SQL in C | Fast Forward | Next |
In Section 31.4 you saw how you can execute SQL statements from an embedded SQL program. Some of those statements only used fixed values and did not provide a way to insert user-supplied values into statements or have the program process the values returned by the query. Those kinds of statements are not really useful in real applications. This section explains in detail how you can pass data between your C program and the embedded SQL statements using a simple mechanism called host variables. In an embedded SQL program we consider the SQL statements to be guests in the C program code which is the host language. Therefore the variables of the C program are called host variables.
Passing data between the C program and the SQL statements is particularly simple in embedded SQL. Instead of having the program paste the data into the statement, which entails various complications, such as properly quoting the value, you can simply write the name of a C variable into the SQL statement, prefixed by a colon. For example:
EXEC SQL INSERT INTO sometable VALUES (:v1, 'foo', :v2);
This statements refers to two C variables named v1 and v2 and also uses a regular SQL string literal, to illustrate that you are not restricted to use one kind of data or the other.
This style of inserting C variables in SQL statements works anywhere a value expression is expected in an SQL statement.
To pass data from the program to the database, for example as parameters in a query, or to pass data from the database back to the program, the C variables that are intended to contain this data need to be declared in specially marked sections, so the embedded SQL preprocessor is made aware of them.
This section starts with
EXEC SQL BEGIN DECLARE SECTION;
and ends with
EXEC SQL END DECLARE SECTION;
Between those lines, there must be normal C variable declarations, such as
int x = 4; char foo[16], bar[16];
As you can see, you can optionally assign an initial value to the variable. The variable's scope is determined by the location of its declaring section within the program. You can also declare variables with the following syntax which implicitly creates a declare section:
EXEC SQL int i = 4;
You can have as many declare sections in a program as you like.
The declarations are also echoed to the output file as normal C variables, so there's no need to declare them again. Variables that are not intended to be used in SQL commands can be declared normally outside these special sections.
The definition of a structure or union also must be listed inside a DECLARE section. Otherwise the preprocessor cannot handle these types since it does not know the definition.
As a host variable you can also use arrays, typedefs, structs and pointers. Moreover there are special types of host variables that exist only in ECPG.
A few examples on host variables:
One of the most common uses of an array declaration is probably the allocation of a char array as in
EXEC SQL BEGIN DECLARE SECTION;
char str[50];
EXEC SQL END DECLARE SECTION;
Note that you have to take care of the length for yourself. If you use this host variable as the target variable of a query which returns a string with more than 49 characters, a buffer overflow occurs.
Use the typedef keyword to map new types to already existing types.
EXEC SQL BEGIN DECLARE SECTION;
typedef char mychartype[40];
typedef long serial_t;
EXEC SQL END DECLARE SECTION;
Note that you could also use
EXEC SQL TYPE serial_t IS long;
This declaration does not need to be part of a declare section.
You can declare pointers to the most common types. Note however that you can not use pointers as target variables of queries without auto-allocation. See Section 31.10 for more information on auto-allocation.
EXEC SQL BEGIN DECLARE SECTION;
int *intp;
char **charp;
EXEC SQL END DECLARE SECTION;
ECPG contains some special types that help you to interact easily with data from the SQL server. For example it has implemented support for the varchar, numeric, date, timestamp, and interval types. Section 31.8 contains basic functions to deal with those types, such that you do not need to send a query to the SQL server just for adding an interval to a timestamp for example.
The special type VARCHAR is converted into a named struct for every variable. A declaration like
VARCHAR var[180];
is converted into
struct varchar_var { int len; char arr[180]; } var;
This structure is suitable for interfacing with SQL datums of type varchar.
Now you should be able to pass data generated by your program into an SQL command. But how do you retrieve the results of a query? For that purpose, embedded SQL provides special variants of the usual commands SELECT and FETCH. These commands have a special INTO clause that specifies which host variables the retrieved values are to be stored in.
Here is an example:
/* * assume this table: * CREATE TABLE test1 (a int, b varchar(50)); */ EXEC SQL BEGIN DECLARE SECTION; int v1; VARCHAR v2; EXEC SQL END DECLARE SECTION; ... EXEC SQL SELECT a, b INTO :v1, :v2 FROM test;
So the INTO clause appears between the select list and the FROM clause. The number of elements in the select list and the list after INTO (also called the target list) must be equal.
Here is an example using the command FETCH:
EXEC SQL BEGIN DECLARE SECTION;
int v1;
VARCHAR v2;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL DECLARE foo CURSOR FOR SELECT a, b FROM test;
...
do {
...
EXEC SQL FETCH NEXT FROM foo INTO :v1, :v2;
...
} while (...);
Here the INTO clause appears after all the normal clauses.
Both of these methods only allow retrieving one row at a time. If you need to process result sets that potentially contain more than one row, you need to use a cursor, as shown in the second example.
The examples above do not handle null values. In fact, the retrieval examples will raise an error if they fetch a null value from the database. To be able to pass null values to the database or retrieve null values from the database, you need to append a second host variable specification to each host variable that contains data. This second host variable is called the indicator and contains a flag that tells whether the datum is null, in which case the value of the real host variable is ignored. Here is an example that handles the retrieval of null values correctly:
EXEC SQL BEGIN DECLARE SECTION; VARCHAR val; int val_ind; EXEC SQL END DECLARE SECTION: ... EXEC SQL SELECT b INTO :val :val_ind FROM test1;
The indicator variable val_ind will be zero if the value was not null, and it will be negative if the value was null.
The indicator has another function: if the indicator value is positive, it means that the value is not null, but it was truncated when it was stored in the host variable.