34.4. Using Host Variables#
InSection 34.3 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 calledhost variables. In an embedded SQL program we consider the SQL statements to beguests in the C program code which is thehost language. Therefore the variables of the C program are calledhost variables.
Another way to exchange values between Postgres Pro backends and ECPG applications is the use of SQL descriptors, described inSection 34.7.
34.4.1. Overview#
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 statement refers to two C variables namedv1 andv2 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.
34.4.2. Declare Sections#
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 aDECLARE section. Otherwise the preprocessor cannot handle these types since it does not know the definition.
34.4.3. Retrieving Query Results#
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 commandsSELECT andFETCH. These commands have a specialINTO clause that specifies which host variables the retrieved values are to be stored in.SELECT is used for a query that returns only single row, andFETCH is used for a query that returns multiple rows, using a cursor.
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 theINTO clause appears between the select list and theFROM clause. The number of elements in the select list and the list afterINTO (also called the target list) must be equal.
Here is an example using the commandFETCH:
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 theINTO clause appears after all the normal clauses.
34.4.4. Type Mapping#
When ECPG applications exchange values between the Postgres Pro server and the C application, such as when retrieving query results from the server or executing SQL statements with input parameters, the values need to be converted between Postgres Pro data types and host language variable types (C language data types, concretely). One of the main points of ECPG is that it takes care of this automatically in most cases.
In this respect, there are two kinds of data types: Some simple Postgres Pro data types, such asinteger andtext, can be read and written by the application directly. Other Postgres Pro data types, such astimestamp andnumeric can only be accessed through special library functions; seeSection 34.4.4.2.
Table 34.1 shows which Postgres Pro data types correspond to which C data types. When you wish to send or receive a value of a given Postgres Pro data type, you should declare a C variable of the corresponding C data type in the declare section.
Table 34.1. Mapping Between Postgres Pro Data Types and C Variable Types
| Postgres Pro data type | Host variable type |
|---|---|
smallint | short |
integer | int |
bigint | long long int |
decimal | decimal[a] |
numeric | numeric[a] |
real | float |
double precision | double |
smallserial | short |
serial | int |
bigserial | long long int |
oid | unsigned int |
character(,varchar(,text | char[,VARCHAR[ |
name | char[NAMEDATALEN] |
timestamp | timestamp[a] |
interval | interval[a] |
date | date[a] |
boolean | bool[b] |
bytea | char *,bytea[ |
[a]This type can only be accessed through special library functions; seeSection 34.4.4.2. [b]declared in | |
34.4.4.1. Handling Character Strings#
To handle SQL character string data types, such asvarchar andtext, there are two possible ways to declare the host variables.
One way is usingchar[], an array ofchar, which is the most common way to handle character data in C.
EXEC SQL BEGIN DECLARE SECTION; char str[50];EXEC SQL END DECLARE SECTION;
Note that you have to take care of the length 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.
The other way is using theVARCHAR type, which is a special type provided by ECPG. The definition on an array of typeVARCHAR is converted into a namedstruct for every variable. A declaration like:
VARCHAR var[180];
is converted into:
struct varchar_var { int len; char arr[180]; } var; The memberarr hosts the string including a terminating zero byte. Thus, to store a string in aVARCHAR host variable, the host variable has to be declared with the length including the zero byte terminator. The memberlen holds the length of the string stored in thearr without the terminating zero byte. When a host variable is used as input for a query, ifstrlen(arr) andlen are different, the shorter one is used.
VARCHAR can be written in upper or lower case, but not in mixed case.
char andVARCHAR host variables can also hold values of other SQL types, which will be stored in their string forms.
34.4.4.2. Accessing Special Data Types#
ECPG contains some special types that help you to interact easily with some special data types from the Postgres Pro server. In particular, it has implemented support for thenumeric,decimal,date,timestamp, andinterval types. These data types cannot usefully be mapped to primitive host variable types (such asint,long long int, orchar[]), because they have a complex internal structure. Applications deal with these types by declaring host variables in special types and accessing them using functions in the pgtypes library. The pgtypes library, described in detail inSection 34.6 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 time stamp for example.
The follow subsections describe these special data types. For more details about pgtypes library functions, seeSection 34.6.
34.4.4.2.1. timestamp, date#
Here is a pattern for handlingtimestamp variables in the ECPG host application.
First, the program has to include the header file for thetimestamp type:
#include <pgtypes_timestamp.h>
Next, declare a host variable as typetimestamp in the declare section:
EXEC SQL BEGIN DECLARE SECTION;timestamp ts;EXEC SQL END DECLARE SECTION;
And after reading a value into the host variable, process it using pgtypes library functions. In following example, thetimestamp value is converted into text (ASCII) form with thePGTYPEStimestamp_to_asc() function:
EXEC SQL SELECT now()::timestamp INTO :ts;printf("ts = %s\n", PGTYPEStimestamp_to_asc(ts));This example will show some result like following:
ts = 2010-06-27 18:03:56.949343
In addition, the DATE type can be handled in the same way. The program has to includepgtypes_date.h, declare a host variable as the date type and convert a DATE value into a text form usingPGTYPESdate_to_asc() function. For more details about the pgtypes library functions, seeSection 34.6.
34.4.4.2.2. interval#
The handling of theinterval type is also similar to thetimestamp anddate types. It is required, however, to allocate memory for aninterval type value explicitly. In other words, the memory space for the variable has to be allocated in the heap memory, not in the stack memory.
Here is an example program:
#include <stdio.h>#include <stdlib.h>#include <pgtypes_interval.h>intmain(void){EXEC SQL BEGIN DECLARE SECTION; interval *in;EXEC SQL END DECLARE SECTION; EXEC SQL CONNECT TO testdb; EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT; in = PGTYPESinterval_new(); EXEC SQL SELECT '1 min'::interval INTO :in; printf("interval = %s\n", PGTYPESinterval_to_asc(in)); PGTYPESinterval_free(in); EXEC SQL COMMIT; EXEC SQL DISCONNECT ALL; return 0;}34.4.4.2.3. numeric, decimal#
The handling of thenumeric anddecimal types is similar to theinterval type: It requires defining a pointer, allocating some memory space on the heap, and accessing the variable using the pgtypes library functions. For more details about the pgtypes library functions, seeSection 34.6.
No functions are provided specifically for thedecimal type. An application has to convert it to anumeric variable using a pgtypes library function to do further processing.
Here is an example program handlingnumeric anddecimal type variables.
#include <stdio.h>#include <stdlib.h>#include <pgtypes_numeric.h>EXEC SQL WHENEVER SQLERROR STOP;intmain(void){EXEC SQL BEGIN DECLARE SECTION; numeric *num; numeric *num2; decimal *dec;EXEC SQL END DECLARE SECTION; EXEC SQL CONNECT TO testdb; EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT; num = PGTYPESnumeric_new(); dec = PGTYPESdecimal_new(); EXEC SQL SELECT 12.345::numeric(4,2), 23.456::decimal(4,2) INTO :num, :dec; printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 0)); printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 1)); printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 2)); /* Convert decimal to numeric to show a decimal value. */ num2 = PGTYPESnumeric_new(); PGTYPESnumeric_from_decimal(dec, num2); printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 0)); printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 1)); printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 2)); PGTYPESnumeric_free(num2); PGTYPESdecimal_free(dec); PGTYPESnumeric_free(num); EXEC SQL COMMIT; EXEC SQL DISCONNECT ALL; return 0;}34.4.4.2.4. bytea#
The handling of thebytea type is similar to that ofVARCHAR. The definition on an array of typebytea is converted into a named struct for every variable. A declaration like:
bytea var[180];
is converted into:
struct bytea_var { int len; char arr[180]; } var; The memberarr hosts binary format data. It can also handle'\0' as part of data, unlikeVARCHAR. The data is converted from/to hex format and sent/received by ecpglib.
Note
bytea variable can be used only whenbytea_output is set tohex.
34.4.4.3. Host Variables with Nonprimitive Types#
As a host variable you can also use arrays, typedefs, structs, and pointers.
34.4.4.3.1. Arrays#
There are two use cases for arrays as host variables. The first is a way to store some text string inchar[] orVARCHAR[], as explained inSection 34.4.4.1. The second use case is to retrieve multiple rows from a query result without using a cursor. Without an array, to process a query result consisting of multiple rows, it is required to use a cursor and theFETCH command. But with array host variables, multiple rows can be received at once. The length of the array has to be defined to be able to accommodate all rows, otherwise a buffer overflow will likely occur.
Following example scans thepg_database system table and shows all OIDs and names of the available databases:
intmain(void){EXEC SQL BEGIN DECLARE SECTION; int dbid[8]; char dbname[8][16]; int i;EXEC SQL END DECLARE SECTION; memset(dbname, 0, sizeof(char)* 16 * 8); memset(dbid, 0, sizeof(int) * 8); EXEC SQL CONNECT TO testdb; EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT; /* Retrieve multiple rows into arrays at once. */ EXEC SQL SELECT oid,datname INTO :dbid, :dbname FROM pg_database; for (i = 0; i < 8; i++) printf("oid=%d, dbname=%s\n", dbid[i], dbname[i]); EXEC SQL COMMIT; EXEC SQL DISCONNECT ALL; return 0;}This example shows following result. (The exact values depend on local circumstances.)
oid=1, dbname=template1oid=11510, dbname=template0oid=11511, dbname=postgresoid=313780, dbname=testdboid=0, dbname=oid=0, dbname=oid=0, dbname=
34.4.4.3.2. Structures#
A structure whose member names match the column names of a query result, can be used to retrieve multiple columns at once. The structure enables handling multiple column values in a single host variable.
The following example retrieves OIDs, names, and sizes of the available databases from thepg_database system table and using thepg_database_size() function. In this example, a structure variabledbinfo_t with members whose names match each column in theSELECT result is used to retrieve one result row without putting multiple host variables in theFETCH statement.
EXEC SQL BEGIN DECLARE SECTION; typedef struct { int oid; char datname[65]; long long int size; } dbinfo_t; dbinfo_t dbval;EXEC SQL END DECLARE SECTION; memset(&dbval, 0, sizeof(dbinfo_t)); EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database; EXEC SQL OPEN cur1; /* when end of result set reached, break out of while loop */ EXEC SQL WHENEVER NOT FOUND DO BREAK; while (1) { /* Fetch multiple columns into one structure. */ EXEC SQL FETCH FROM cur1 INTO :dbval; /* Print members of the structure. */ printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, dbval.size); } EXEC SQL CLOSE cur1;This example shows following result. (The exact values depend on local circumstances.)
oid=1, datname=template1, size=4324580oid=11510, datname=template0, size=4243460oid=11511, datname=postgres, size=4324580oid=313780, datname=testdb, size=8183012
Structure host variables“absorb” as many columns as the structure as fields. Additional columns can be assigned to other host variables. For example, the above program could also be restructured like this, with thesize variable outside the structure:
EXEC SQL BEGIN DECLARE SECTION; typedef struct { int oid; char datname[65]; } dbinfo_t; dbinfo_t dbval; long long int size;EXEC SQL END DECLARE SECTION; memset(&dbval, 0, sizeof(dbinfo_t)); EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database; EXEC SQL OPEN cur1; /* when end of result set reached, break out of while loop */ EXEC SQL WHENEVER NOT FOUND DO BREAK; while (1) { /* Fetch multiple columns into one structure. */ EXEC SQL FETCH FROM cur1 INTO :dbval, :size; /* Print members of the structure. */ printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, size); } EXEC SQL CLOSE cur1;34.4.4.3.3. Typedefs#
Use thetypedef 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; that is, you can also write typedefs as normal C statements.
Any word you declare as atypedef cannot be used as an SQL keyword inEXEC SQL commands later in the same program. For example, this won't work:
EXEC SQL BEGIN DECLARE SECTION; typedef int start;EXEC SQL END DECLARE SECTION;...EXEC SQL START TRANSACTION;
ECPG will report a syntax error forSTART TRANSACTION, because it no longer recognizesSTART as an SQL keyword, only as a typedef. (If you have such a conflict, and renaming the typedef seems impractical, you could write the SQL command usingdynamic SQL.)
Note
InPostgreSQL releases before v16, use of SQL keywords as typedef names was likely to result in syntax errors associated with use of the typedef itself, rather than use of the name as an SQL keyword. The new behavior is less likely to cause problems when an existing ECPG application is recompiled in a newPostgreSQL release with new keywords.
34.4.4.3.4. Pointers#
You can declare pointers to the most common types. Note however that you cannot use pointers as target variables of queries without auto-allocation. SeeSection 34.7 for more information on auto-allocation.
EXEC SQL BEGIN DECLARE SECTION; int *intp; char **charp;EXEC SQL END DECLARE SECTION;
34.4.5. Handling Nonprimitive SQL Data Types#
This section contains information on how to handle nonscalar and user-defined SQL-level data types in ECPG applications. Note that this is distinct from the handling of host variables of nonprimitive types, described in the previous section.
34.4.5.1. Arrays#
Multi-dimensional SQL-level arrays are not directly supported in ECPG. One-dimensional SQL-level arrays can be mapped into C array host variables and vice-versa. However, when creating a statement ecpg does not know the types of the columns, so that it cannot check if a C array is input into a corresponding SQL-level array. When processing the output of an SQL statement, ecpg has the necessary information and thus checks if both are arrays.
If a query accesseselements of an array separately, then this avoids the use of arrays in ECPG. Then, a host variable with a type that can be mapped to the element type should be used. For example, if a column type is array ofinteger, a host variable of typeint can be used. Also if the element type isvarchar ortext, a host variable of typechar[] orVARCHAR[] can be used.
Here is an example. Assume the following table:
CREATE TABLE t3 ( ii integer[]);testdb=> SELECT * FROM t3; ii------------- {1,2,3,4,5}(1 row) The following example program retrieves the 4th element of the array and stores it into a host variable of typeint:
EXEC SQL BEGIN DECLARE SECTION;int ii;EXEC SQL END DECLARE SECTION;EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[4] FROM t3;EXEC SQL OPEN cur1;EXEC SQL WHENEVER NOT FOUND DO BREAK;while (1){ EXEC SQL FETCH FROM cur1 INTO :ii ; printf("ii=%d\n", ii);}EXEC SQL CLOSE cur1;This example shows the following result:
ii=4
To map multiple array elements to the multiple elements in an array type host variables each element of array column and each element of the host variable array have to be managed separately, for example:
EXEC SQL BEGIN DECLARE SECTION;int ii_a[8];EXEC SQL END DECLARE SECTION;EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[1], ii[2], ii[3], ii[4] FROM t3;EXEC SQL OPEN cur1;EXEC SQL WHENEVER NOT FOUND DO BREAK;while (1){ EXEC SQL FETCH FROM cur1 INTO :ii_a[0], :ii_a[1], :ii_a[2], :ii_a[3]; ...}Note again that
EXEC SQL BEGIN DECLARE SECTION;int ii_a[8];EXEC SQL END DECLARE SECTION;EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii FROM t3;EXEC SQL OPEN cur1;EXEC SQL WHENEVER NOT FOUND DO BREAK;while (1){ /* WRONG */ EXEC SQL FETCH FROM cur1 INTO :ii_a; ...}would not work correctly in this case, because you cannot map an array type column to an array host variable directly.
Another workaround is to store arrays in their external string representation in host variables of typechar[] orVARCHAR[]. For more details about this representation, seeSection 8.15.2. Note that this means that the array cannot be accessed naturally as an array in the host program (without further processing that parses the text representation).
34.4.5.2. Composite Types#
Composite types are not directly supported in ECPG, but an easy workaround is possible. The available workarounds are similar to the ones described for arrays above: Either access each attribute separately or use the external string representation.
For the following examples, assume the following type and table:
CREATE TYPE comp_t AS (intval integer, textval varchar(32));CREATE TABLE t4 (compval comp_t);INSERT INTO t4 VALUES ( (256, 'Postgres Pro') );
The most obvious solution is to access each attribute separately. The following program retrieves data from the example table by selecting each attribute of the typecomp_t separately:
EXEC SQL BEGIN DECLARE SECTION;int intval;varchar textval[33];EXEC SQL END DECLARE SECTION;/* Put each element of the composite type column in the SELECT list. */EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;EXEC SQL OPEN cur1;EXEC SQL WHENEVER NOT FOUND DO BREAK;while (1){ /* Fetch each element of the composite type column into host variables. */ EXEC SQL FETCH FROM cur1 INTO :intval, :textval; printf("intval=%d, textval=%s\n", intval, textval.arr);}EXEC SQL CLOSE cur1; To enhance this example, the host variables to store values in theFETCH command can be gathered into one structure. For more details about the host variable in the structure form, seeSection 34.4.4.3.2. To switch to the structure, the example can be modified as below. The two host variables,intval andtextval, become members of thecomp_t structure, and the structure is specified on theFETCH command.
EXEC SQL BEGIN DECLARE SECTION;typedef struct{ int intval; varchar textval[33];} comp_t;comp_t compval;EXEC SQL END DECLARE SECTION;/* Put each element of the composite type column in the SELECT list. */EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;EXEC SQL OPEN cur1;EXEC SQL WHENEVER NOT FOUND DO BREAK;while (1){ /* Put all values in the SELECT list into one structure. */ EXEC SQL FETCH FROM cur1 INTO :compval; printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);}EXEC SQL CLOSE cur1; Although a structure is used in theFETCH command, the attribute names in theSELECT clause are specified one by one. This can be enhanced by using a* to ask for all attributes of the composite type value.
...EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).* FROM t4;EXEC SQL OPEN cur1;EXEC SQL WHENEVER NOT FOUND DO BREAK;while (1){ /* Put all values in the SELECT list into one structure. */ EXEC SQL FETCH FROM cur1 INTO :compval; printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);}...This way, composite types can be mapped into structures almost seamlessly, even though ECPG does not understand the composite type itself.
Finally, it is also possible to store composite type values in their external string representation in host variables of typechar[] orVARCHAR[]. But that way, it is not easily possible to access the fields of the value from the host program.
34.4.5.3. User-Defined Base Types#
New user-defined base types are not directly supported by ECPG. You can use the external string representation and host variables of typechar[] orVARCHAR[], and this solution is indeed appropriate and sufficient for many types.
Here is an example using the data typecomplex from the example inSection 36.13. The external string representation of that type is(%f,%f), which is defined in the functionscomplex_in() andcomplex_out() functions inSection 36.13. The following example inserts the complex type values(1,1) and(3,3) into the columnsa andb, and select them from the table after that.
EXEC SQL BEGIN DECLARE SECTION; varchar a[64]; varchar b[64];EXEC SQL END DECLARE SECTION; EXEC SQL INSERT INTO test_complex VALUES ('(1,1)', '(3,3)'); EXEC SQL DECLARE cur1 CURSOR FOR SELECT a, b FROM test_complex; EXEC SQL OPEN cur1; EXEC SQL WHENEVER NOT FOUND DO BREAK; while (1) { EXEC SQL FETCH FROM cur1 INTO :a, :b; printf("a=%s, b=%s\n", a.arr, b.arr); } EXEC SQL CLOSE cur1;This example shows following result:
a=(1,1), b=(3,3)
Another workaround is avoiding the direct use of the user-defined types in ECPG and instead create a function or cast that converts between the user-defined type and a primitive type that ECPG can handle. Note, however, that type casts, especially implicit ones, should be introduced into the type system very carefully.
For example,
CREATE FUNCTION create_complex(r double, i double) RETURNS complexLANGUAGE SQLIMMUTABLEAS $$ SELECT $1 * complex '(1,0')' + $2 * complex '(0,1)' $$;
After this definition, the following
EXEC SQL BEGIN DECLARE SECTION;double a, b, c, d;EXEC SQL END DECLARE SECTION;a = 1;b = 2;c = 3;d = 4;EXEC SQL INSERT INTO test_complex VALUES (create_complex(:a, :b), create_complex(:c, :d));
has the same effect as
EXEC SQL INSERT INTO test_complex VALUES ('(1,2)', '(3,4)');34.4.6. Indicators#
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 theindicator 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 variableval_ind will be zero if the value was not null, and it will be negative if the value was null. (SeeSection 34.16 to enable Oracle-specific behavior.)
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.
If the argument-r no_indicator is passed to the preprocessorecpg, it works in“no-indicator” mode. In no-indicator mode, if no indicator variable is specified, null values are signaled (on input and output) for character string types as empty string and for integer types as the lowest possible value for type (for example,INT_MIN forint).