Informix Error -251

Reasons / Common Causes

-251 means an ORDER BY or GROUP BY clause used a column-sequence number larger than the actual number of columns in the select list.

1. A typo in the column number.
2. The select list was edited (a column removed) without updating a corresponding ORDER BY/GROUP BY column-number reference that pointed at it or beyond it.
3. Miscounting column positions, especially in a select list containing expressions or aliases that make visual counting less obvious than with plain column names.
4. Confusion between positional numbering and an intended column name — using a number when a name was actually meant.

Solutions / Resolution

1. Check the ORDER BY/GROUP BY clause against the actual select list, per the official guidance, and confirm no item was omitted from the select list.
2. Correct the column number if the select list changed since the clause was written.
3. Consider using explicit column names or aliases in ORDER BY/GROUP BY instead of positional numbers — this avoids the entire class of fragility where a later select-list edit silently breaks a positional reference.

Examples

The straightforward case

SELECT customer_id, name, region FROM customer ORDER BY 5;
-- -251: only 3 columns in the select list, but ORDER BY references
-- column 5

Fix:

SELECT customer_id, name, region FROM customer ORDER BY 3;
-- or, more robustly:
SELECT customer_id, name, region FROM customer ORDER BY region;

A select list edit breaking a positional reference

-- Originally:
SELECT id, name, email, phone FROM customer ORDER BY 4;  -- phone

-- After removing a column during a later edit:
SELECT id, name, email FROM customer ORDER BY 4;
-- -251: phone is gone, and only 3 columns remain

Using ORDER BY email (or whatever the actual intended column is) instead of a positional number avoids this fragility entirely.

Diagnostic Checks

1. Count the columns in the select list and compare against the number referenced in ORDER BY/GROUP BY.
2. Review recent edits to the select list if this appeared unexpectedly on a query that used to work.

Related Errors / Related Topics

• -201 — "A syntax error has occurred." The general SQL-parsing-error family this fits into.

Prefer explicit column names over positional numbers in ORDER BY/GROUP BY — it's the best prevention against this error recurring after future select-list edits.