How to check main blog in a multisite using if statement?

I want to create one extra table filed if it is multisite’s main blog (While plugin activation).

I mean like this

if (main blog) {
  execute this sql;
}
else {
  execute this sql
}

Can anyone help me?

PS: This is the code for “execute this sql”.

If child blog:

$sql = "CREATE TABLE tablename (
                  id bigint(20) NOT NULL AUTO_INCREMENT,
                  uid bigint(20) NOT NULL,
                  type VARCHAR(256) NOT NULL,
                  data TEXT NOT NULL,
                  credits bigint(20) NOT NULL,
                  timestamp bigint(20) NOT NULL,
                  UNIQUE KEY id (id)
                );";

If main blog:

$sql = "CREATE TABLE tablename (
              id bigint(20) NOT NULL AUTO_INCREMENT,
              uid bigint(20) NOT NULL,
              type VARCHAR(256) NOT NULL,
              data TEXT NOT NULL,
              credits bigint(20) NOT NULL,
                      globalcredits bigint(20) NOT NULL,
              timestamp bigint(20) NOT NULL,
              UNIQUE KEY id (id)
            );";