Explanation about a code fragment

Question

I see the following code fragment in a legacy application that accesses Informix through C. Can anyone explain what the SQL in the code is trying to achieve? Thanks.

EXEC SQL BEGIN DECLARE SECTION;
int i_tableref;
EXEC SQL END DECLARE SECTION;

    /* Some code here */

if (!i_sel_ref)
{
    exec sql begin declare section;
    const char *sql1 = 
        "select refer_num.nextval from table ( SET{''} )";
    exec sql end declare section;
    exec sql prepare oref_sel_fid from :sql1;
    if ( sqlca.sqlcode != SQL_OK )
    {
         /* some code */
    }
    /* More code */
}

Answer 1

I believe it is obtaining the next value from a database sequence called refer_num. Sequences are a way of generating values for numeric unique identifiers - a bit like IDENTITY columns in some DBMSs. I don't know Informix, but my guess is that "table ( SET{''} )" is a way of generating a pseudo-table with 1 row so that you can perform a select statement that doesn't actually need to access any real database table. Oracle has a special table called DUAL for this purpose, and this would be a common sight in Oracle:

select refer_num.nextval from dual;

Answer 2

for getting next value of a sequence generator in Informix:

select your_seq_generator_name.nextval from table(set{1});