How to get COUNT(*) value from sqlite3_exec()? - c

Question

In my C program I have to check if count of a table in database is one or zero and to do that i am executing query as follows:
char *sql = "SELECT COUNT(*) FROM sqlite_master WHERE type='table' AND name=family;";
int table_count = sqlite3_exec(db, sql, 0, 0, &err_msg);
printf("\n%d\n", table_count);
I'm expecting table_count to be 1 as only one table exists with name family but printf outputs table_count as '21' which is incorrect. How can we get the COUNT(*) value from C/C++ API in C program the right/correct way?

Answer 1

After reading SQLite Documentation and following other kind implicit/explicit suggestions in the comments on the question, I have realized my mistakes in that code snippet quoted in the question.
Mistake 1:
I did not implement callback function to receive the result set after the SQL query gets executed. [Have implemented this callback: see checkTable_Callback in code below]
Mistake 2:
That output of '21' is actually the error code and as per the SQLite documentation that error code corresponds to SQLite_MISUSE Which was being generated, perhaps, because I was using a separate function to open my test database file and instance of that opened database, i assume, stayed inside that openDb function, and when i used another function checkTableCount from where i took that messy snippet to quote in my question, there db instance perhaps was null, hence 21. Experts can elaborate further if that's why i was receiving error code 21. Anyways, now i have fixed that function and made that openDb return an opened db instance (better word?) and now 21 error is gone. [see code below]
Here is fixed and 'adapted-for-my-case' code
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "sqlite3.h" /* sqlite3.dll and sqlite3.h both reside in
my <program_source.c>'s folder */
static int checkTable_Callback(
void *unnecessary_here,
int number_of_columns_in_result_row, /* will always be 1 in this case */
char **value_of_count, /* will be either 0 or 1 in this case */
char **label_for_count) { /* will be COUNT(*) normally,
but modified via 'AS table_tablename' in this case*/
printf("COUNT(*) %s\t=>\t%s\n", label_for_count[0], value_of_count[0] );
return 0;
} // end of checkTable_Callback()
char * build_sql(const char *sql_partA, const char *sql_partB) {
size_t size = strlen(sql_partA) + strlen(sql_partB);
char *sql_final = malloc(size + 1);
strcpy(sql_final, sql_partA);
strcat(sql_final, sql_partB);
return sql_final; /* allocated memory to be freed at the end of calling function */
} // end of build_sql()
checkTableCount(sqlite3 *db, char *tablename) {
char *sql = build_sql(
build_sql(
build_sql(
build_sql(
"SELECT COUNT(*) AS table_",
tablename),
" FROM sqlite_master WHERE type='table' AND name='"),
tablename),
"';");
sqlite3_exec(db, sql, checkTable_Callback, 0, NULL);
/* error checking sacrificed for brevity of sample */
free(sql);
}// end of checkTableCount()
sqlite3 * openDb(char * db_name){
sqlite3 *db;
int result_code = sqlite3_open(db_name, &db);
if( result_code != 0 )
fprintf(stderr, "\tError: %s\n\n", sqlite3_errmsg(db));
return db;
} // end of openDb()
int main() {
sqlite3 * db = openDb("testing.db"); /* testing.db already has one table 'family'*/
checkTableCount(db, "family");
checkTableCount(db, "fam"); /* no such table exist */
sqlite3_close(db);
return 0;
} // end of main()
Now this quoted 'adapted-for-my-case' code rightly and correctly outputs the COUNT(*) as follows:
OUTPUT
COUNT(*) table_family => 1
COUNT(*) table_fam => 0
Note that I didn't bother to write a for-loop inside my callback function named checkTable_Callback to iterate through columns as shown in the official sample of callback function on this page because of the fact that our expected result row is certainly going to be only one containing only one column with label modified, via 'AS' clause, into 'table_tablename'. If not modified via 'AS clause', the returned column label would be 'COUNT(*)' in the result row.