无法使用C将列添加到现有表

Question

The following program tries to insert a new row and a new column to an already existing database with a table "people" containing four cols (id, lastname, firstname, phonenumber). While the row gets inserted successfully, the column is not getting added.

    #include <stdio.h>
    #include <stdlib.h>
    #include <libpq-fe.h>
    #include <string.h>

    int main() 
    {
     PGconn *conn;
     PGresult *res;
     int rec_count;
     int row;
     int col;

     conn = PQconnectdb("dbname=test host=localhost user=abc1 password=xyz1");

         if(PQstatus(conn) == CONNECTION_BAD) {
             puts("We were unable to connect to the database");
             exit(0);
         }

     res = PQexec(conn,"INSERT INTO people VALUES (5, 'XXX', 'YYY', '7633839276');");
     if(PQresultStatus(res) != PGRES_COMMAND_OK) {
        fprintf(stderr, "Insertion Failed1: %s", PQerrorMessage(conn));
        PQclear(res);
         }
     else
        printf("Successfully inserted value in Table..... \n");

         res = PQexec(conn,"update people set phonenumber=\'5055559999\' where id=3");
     if(PQresultStatus(res) != PGRES_COMMAND_OK) {
        fprintf(stderr, "Insertion Failed2: %s", PQerrorMessage(conn));
        PQclear(res);
         }

     res = PQexec(conn, "ALTER TABLE people ADD comment VARCHAR(100) DEFAULT 'TRUE'");
     if(PQresultStatus(res) != PGRES_COMMAND_OK) {
        fprintf(stderr, "Insertion Failed3: %s", PQerrorMessage(conn));
        PQclear(res);
         }

  rec_count = PQntuples(res);

      printf("We received %d records.\n", rec_count);
      puts("==========================");

      for(row=0; row<rec_count; row++) {
          for(col=0; col<3; col++) {
              printf("%s\t", PQgetvalue(res, row, col));
           }
           puts("");
        }

      puts("==========================");

      PQclear(res);

      PQfinish(conn);

      return 0;
    }

The program after being compiled and linked gives the following output:

    $ ./test
    Successfully inserted value in Table..... 
    We received 0 records.
    ==========================
    ==========================

In the postgresql environment, the table "people" is updated with an extra row and a column containing "TRUE".

This is my first program with C embedded with postgresql. Please help!!

Answer 1

This line:

 res = PQexec(conn, "EXEC('UPDATE people SET comment = ''TRUE'';');");

should be:

 res = PQexec(conn, "UPDATE people SET comment = 'TRUE'");

The EXEC syntax is part of embedded SQL in C with ECPG . It's not to be combined with PQexec from the C libpq library , which is clearly what you're using given the rest of the source code.

Also the result of an UPDATE with no RETURNING clause has PQresultStatus(res)==PGRES_COMMAND_OK , not PGRES_TUPLES_OK

PGRES_COMMAND_OK

 Successful completion of a command returning no data. 

See Command Execution Functions

You also want to test any PGresult returned by PQexec , not only the result of your last query, because any query may fail.

Answer 2

How can you retrieve data without doing a SELECT statement ?

Never used PQexec, but i guess the code to add might be like this:

 res = PQexec(conn, "ALTER TABLE people ADD comment VARCHAR(100) DEFAULT 'TRUE'");
 if(PQresultStatus(res) != PGRES_COMMAND_OK) {
    fprintf(stderr, "Insertion Failed3: %s", PQerrorMessage(conn));
    PQclear(res);
 }

//Add PQexec and select statement here.
//Something "like" this.
 res = PQexec(conn, "SELECT * FROM people;");
 if(PQresultStatus(res) != PGRES_TUPLES_OK) {
    fprintf(stderr, "Select Failed: %s", PQerrorMessage(conn));
    PQclear(res);
 }

  rec_count = PQntuples(res);

  printf("We received %d records.\n", rec_count);