how to get sub string from another string in ( C )

Question

i write a function for find table name in sql query string.

my code

char* SQLParser_GetTable(char *query)
{
char *str = "";
char *FROM="FROM";    

if(strstr(query, FROM))
{
    char *e;
    int index;      
    e = strchr(query, 'F');
    index = (int)(e - FROM);

    str=substring(str,index+4,5);
}

return  str;
}

main.c

     query = "SELECT * FROM TABLE1";
     char *tbl=SQLParser_GetTable(query);

but this code retun full string not table name.

MY code must return "TABLE1".

What are you expecting the following two lines to do: e = strchr(query, 'F'); index = (int)(e - FROM);? — NPE
– NPE, Commented Jan 1, 2015 at 12:35

Iharob Al Asimi · Accepted Answer · 2015-01-01 12:44:03Z

1

I don't know what substring() is but this code does what you think yours does

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *SQLParser_GetTable(char *query)
{
    char *str;
    char *FROM = "FROM ";

    /* point to the start of FORM */
    if ((str = strstr(query, FROM)) == NULL)
        return NULL;
    /* If there is only one space between FROM and TABLE1 point to it */
    str = strchr(str, ' ');
    if (str == NULL)
        return NULL;
    /* move past the ' ' character */
    str += 1;
    /* return a copy of the string */
    return strdup(str);
}

int main ()
{
    char *table = SQLParser_GetTable("SELECT * FROM TABLE1");
    if (table != NULL)
    {
        printf("%s\n", table);
        free(table);
    }
    return 0;
}

notice that this is not robust at all since there can be any number of whitespaces between FROM and the table name.

edited Jan 1, 2015 at 12:44

answered Jan 1, 2015 at 12:39

Iharob Al Asimi

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2 Comments

Tarrah Arshad Over a year ago

how to can get columns and where condition .ex: select col1,col2 from table1 where col1=1 ? and save in array or linked list ? can i contact to me tehhitech [at] live .com . i have many project to you now.

Iharob Al Asimi Over a year ago

I wouldn't recommend that, you should use a parser generator. see bison and flex otherwise you should use a state machine and write a tokenizer, since sql sentences can have any number of whitespaces and there are many other situations to handle too. You need to write a lexer and specify a grammar, that can be done with the programs in the links, not in that order. And sure, [email protected]

Vlad from Moscow · Accepted Answer · 2015-01-01 13:53:57Z

0

I have written the following long function but compared with functions in other answers it is the only one that is more or less correct.:). The function does not take into account the case of letters.:)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

char * SQLParser_GetTable( const char *query )
{
    const char FROM[] = "FROM";    
    char *p;
    int found;

    found = ( p = strstr( query, FROM ) ) != NULL;
    found = found && ( p == query || isblank( ( unsigned char )p[-1] ) );
    found = found && ( *( p += sizeof( FROM ) - 1 ) == '\0' ||
                       isblank( ( unsigned char )p[0] ) );

    if ( found )
    {
        const char *q = p;
        size_t n = 0;

        while ( isblank( ( unsigned char )*q ) ) ++q;
        while ( q[n] && !isblank( ( unsigned char )q[n] ) ) ++n;

        p = malloc( ( n + 1 ) * sizeof( char ) );

        memcpy( p, q, n );
        p[n] = '\0';
    }
    else
    {
        p = NULL;
    }

    return p;
}

int main(void) 
{
    char *query = "SELECT * FROM TABLE1";
    char *p = SQLParser_GetTable( query );

    if ( p ) puts( p );

    free( p );

    return 0;
}

The output is

TABLE1

edited Jan 1, 2015 at 13:53

answered Jan 1, 2015 at 13:32

Vlad from Moscow

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4 Comments

Iharob Al Asimi Over a year ago

This is a good approach, though your code is not very readable and you should use isspace instead, because in SQL newlines are also ignored. +1.

Vlad from Moscow Over a year ago

@iharob Only now I have found that the code contains a defect. For example a SQL statement can contain more than one "FROM" characters and the first one that will be found is not necessary the right "FROM".:) So the search should be continued until there is no any "FROM".

Iharob Al Asimi Over a year ago

And there are many more issues, that's why I recommended a parser generator.

Tarrah Arshad Over a year ago

than you Vlad from moscow about better answer .

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