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/ Chapter 9 · Query Debugging and Common Mistakes
Lesson 9.2 · garden_shop
Lesson 9.2

Debugging SQL Queries Step by Step

When a query surprises you, shrink it. Inspect the data, check one piece at a time, and the bug surfaces.

Not every bug throws an error. Sometimes a query runs perfectly and returns nothing, which is its own kind of clue. The fix is to debug in small steps: strip the query back, look at the actual data, and add conditions one at a time.

The query below expects shipped orders but comes back empty. Before changing it blindly, inspect what's really in the status column. String comparisons are case-sensitive, so a capital letter is all it takes to match nothing.

Pattern 
-- when a result is empty, inspect the real values:
select distinct status from orders;

-- ...then fix the filter to match
Schema · Garden Shop Table · orders 6 columns · 24 rows
Table · orders

One row per order. Unshipped orders have a null shipped_date.

6 columns · 24 rows
order_id int customer_id int order_date date shipped_date date status text coupon_code text
Your task

The starter returns zero rows. Figure out why, then fix it so it lists every shipped order: order_id and status, sorted by order_id.

·

Run your query to inspect the result.

Expected answer
  • Columns: order_id, status.
  • Rows: 17 shipped orders.
  • String comparisons are case-sensitive, so 'Shipped' matched nothing.
Next · 9.3 Wrong answers without errors →

Related

Reference
SQL Query Formatting Guide

Indentation, casing, and aliasing conventions.

Mistake
COUNT or SUM Too High After a JOIN

One-to-many joins can inflate totals.

Error
SQL Error Messages Explained

Paste an error message and find the fix, from GROUP BY to constraint failures.

Mistake
Why My Query Returns No Rows

Debug filters that match nothing.

Error
Fix “Column Must Appear in the GROUP BY Clause” in SQL

Group or aggregate every selected column.

Mistake
Why a LEFT JOIN Turns Into an INNER JOIN

Right-table WHERE filters remove unmatched rows.