Re: conditional IF statements in postgresql - Mailing list pgsql-general

Hi Pujol,

Thanks a ton for your help!!

I was missing the semicolon and it works fine now..

Thanks,

Madhurima

On Mon, Jul 7, 2014 at 11:09 AM, Madhurima Das <madhurima.das@gmail.com> wrote:

I just checked that anything after the line 

ALTER TABLE protein_sequence ADD comment VARCHAR(500) \n\

does not work and gives the same syntax error as above.

On Mon, Jul 7, 2014 at 10:56 AM, Madhurima Das <madhurima.das@gmail.com> wrote:

Thanks once again.. However, I get a error after running the program as:

Adding col to table (ALTER) Failed: ERROR:  syntax error at or near "ELSE"

LINE 5: ELSE

        ^

Can you provide some suggestions.

On Mon, Jul 7, 2014 at 10:46 AM, Pujol Mathieu <mathieu.pujol@realfusio.com> wrote:

Le 07/07/2014 13:44, Pujol Mathieu a écrit :

Le 07/07/2014 12:48, Albe Laurenz a écrit :

Madhurima Das wrote:

I am writing a C program to access a PostgreSQL database, where
I add a column if it doesn't exists in the table
or, update the column, if the column already exits.
Please suggest how to work with the conditional statements.
N.B. I wrote the following:

res = PQexec(conn, "IF COL_LENGTH('protein_sequence','comment') IS NULL");
PQclear(res);
if(res)
  {
      res = PQexec(conn, "ALTER TABLE protein_sequence ADD comment VARCHAR(500)");
      PQclear(res);
  }
  else
  {
      res = PQexec(conn, "UPDATE TABLE protein_sequence ADD comment VARCHAR(500)");
       PQclear(res);
  }

Is the code logically correct??

No, that doesn't make any sense.
The statement sent with PQexec must be a legal SQL statement.

You could do it like this:

/* try the update */
res = PQexec(conn, "UPDATE protein_sequence SET comment = ... WHERE ...");
if (!res) {
     /* out of memory, error out */
}
r = PQresultStatus(res);
PQclear(res);
if (r == PGRES_COMMAND_OK) {
     return;  /* UPDATE ok */
} else if (r != PGRES_NONFATAL_ERROR) {
     /* unexpected result, error out */
}

/* add the column */
res = PQexec(conn, "ALTER TABLE protein_sequence ADD comment VARCHAR(500)");
if (!res) {
     /* out of memory, error out */
}
r = PQresultStatus(res);
PQclear(res);
if (r == PGRES_COMMAND_OK) {
     return;  /* ALTER TABLE ok */
} else {
     /* unexpected result, error out */
}

This code is untested.

Yours,
Laurenz Albe

Snippet Hi,
You can do that in a single statement
std::string lStatement;
lStatement += "DO $$\n"; lStatement += "BEGIN\n";
lStatement += "IF COL_LENGTH('protein_sequence','comment') IS NULL THEN\n";
lStatement += "ALTER TABLE protein_sequence ADD comment VARCHAR(500) \n";
lStatement += "ELSE\n";
lStatement += "UPDATE TABLE protein_sequence ADD comment VARCHAR(500)\n";
lStatement += "END IF;\n";
lStatement += "END;\n";
lStatement += "$$;\n";
res = PQexec(conn, lStatement .c_str());
Regards,
Mathieu

Hi,
My answer is a C++ sample.
This looks like you don't have include string header, or use this code if if you are writing pure C program.

const char * lStatement = "\
DO $$\n\
BEGIN\n\
IF COL_LENGTH('protein_sequence','comment') IS NULL THEN\n\
ALTER TABLE protein_sequence ADD comment VARCHAR(500) \n\
ELSE\n\
UPDATE TABLE protein_sequence ADD comment VARCHAR(500)\n\
END IF;\n\
END;\n\
$$;\n"; 

Regards,
Mathieu

Hi,
I'm glad that helps you. Just feew tips for using the mailing list. You should reply to the mailing list pgsql-general@postgresql.org( and not only the person who answers you), so that other users could see that your problem is solve. You should also write your answer at the bottom of the mail to facilitate reading.
Regards,
Mathieu