Non-equi joins in SQL - Step-by-Step Execution

Concept Flow - Non-equi joins

Start with two tables

↓

Apply join condition

↓

Check if condition is non-equality (>, <, >=, <=, !=)

Yes↓

Match rows based on condition

↓

Return combined rows

↓

End

Non-equi joins combine rows from two tables based on conditions other than equality, like greater than or less than.

Execution Sample

SQL

SELECT A.id, B.value
FROM A
JOIN B ON A.score > B.threshold;

This query joins tables A and B where A's score is greater than B's threshold.

Execution Table

Step	Row from A	Row from B	Condition (A.score > B.threshold)	Join Result
1	{id:1, score:50}	{threshold:30, value:'X'}	50 > 30 = True	{id:1, value:'X'}
2	{id:1, score:50}	{threshold:60, value:'Y'}	50 > 60 = False	No
3	{id:2, score:70}	{threshold:30, value:'X'}	70 > 30 = True	{id:2, value:'X'}
4	{id:2, score:70}	{threshold:60, value:'Y'}	70 > 60 = True	{id:2, value:'Y'}
5	{id:3, score:20}	{threshold:30, value:'X'}	20 > 30 = False	No
6	{id:3, score:20}	{threshold:60, value:'Y'}	20 > 60 = False	No
7	End of rows			Query complete

💡 All rows from A compared with all rows from B; join ends when all pairs checked.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
Current A row	None	{id:1, score:50}	{id:1, score:50}	{id:2, score:70}	{id:2, score:70}	{id:3, score:20}	{id:3, score:20}	None
Current B row	None	{threshold:30, value:'X'}	{threshold:60, value:'Y'}	{threshold:30, value:'X'}	{threshold:60, value:'Y'}	{threshold:30, value:'X'}	{threshold:60, value:'Y'}	None
Join Result	Empty	{id:1, value:'X'}	No	{id:2, value:'X'}	{id:2, value:'Y'}	No	No	Final result set

Key Moments - 2 Insights

Why do some row pairs not appear in the join result?

Because the join condition (A.score > B.threshold) is false for those pairs, so they are excluded as shown in execution_table rows 2, 5, and 6.

Is this join the same as an equi-join?

No, equi-joins use equality (=) conditions, but here we use a greater-than (>) condition, which is why some pairs match differently, as seen in the condition column.

Visual Quiz - 3 Questions

Look at the execution table, what is the join result at step 4?

A {id:2, value:'X'}

B No

C {id:2, value:'Y'}

D {id:1, value:'X'}

💡 Hint

Check the 'Join Result' column in row 4 of the execution_table.

At which step does the condition first evaluate to false?

A Step 3

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Condition' column in the execution_table to find the first 'False'.

If A.score was always less than B.threshold, what would the join result be?

A Empty result set

B Only rows where scores are equal

C All rows from A joined with all rows from B

D Only rows where scores are greater

💡 Hint

Refer to the variable_tracker and execution_table where condition is false and no rows join.

Concept Snapshot

Non-equi joins use conditions other than '=' to join tables.
Syntax example: JOIN ... ON A.col > B.col
Returns rows where condition is true.
Useful for ranges, inequalities, and complex matching.
Unlike equi-joins, matches depend on >, <, >=, <=, != conditions.

Full Transcript

Non-equi joins combine rows from two tables based on conditions that are not equality. For example, joining where one column is greater than another. The process starts by taking each row from the first table and comparing it to each row in the second table. If the condition like A.score > B.threshold is true, the rows are joined and included in the result. If false, they are skipped. This is different from equi-joins which only join rows with equal values. The example query joins tables A and B where A's score is greater than B's threshold. Step by step, each pair is checked, and only those pairs meeting the condition appear in the output. This method is useful when you want to join based on ranges or inequalities rather than exact matches.

Practice

1. What is a non-equi join in SQL?

easy

A. A join that uses conditions other than equality, like <, >, or BETWEEN.

B. A join that only matches rows with equal values in both tables.

C. A join that combines all rows from both tables regardless of condition.

D. A join that uses only the AND logical operator in the ON clause.

Solution

1. Step 1: Understand join conditions

   Equi joins use equality (=) to match rows. Non-equi joins use other operators like <, >, or BETWEEN.

2. Step 2: Identify non-equi join definition

   Since non-equi joins match rows based on inequalities or ranges, A join that uses conditions other than equality, like <, >, or BETWEEN. correctly describes this.

3. Final Answer:

   A join that uses conditions other than equality, like <, >, or BETWEEN. -> Option A

4. Quick Check:

   Non-equi join = condition other than = [OK]

Hint: Non-equi joins use <, >, or BETWEEN, not just = [OK]

Common Mistakes:

• Confusing non-equi join with equi join
• Thinking non-equi join matches all rows
• Assuming only AND operator defines non-equi join

2. Which of the following is the correct syntax for a non-equi join using BETWEEN?

easy

A. SELECT * FROM A JOIN B ON A.value IN BETWEEN B.min AND B.max;

B. SELECT * FROM A JOIN B ON A.value = BETWEEN B.min AND B.max;

C. SELECT * FROM A JOIN B ON BETWEEN A.value AND B.min AND B.max;

D. SELECT * FROM A JOIN B ON A.value BETWEEN B.min AND B.max;

Solution

1. Step 1: Recall BETWEEN syntax

   BETWEEN is used as: column BETWEEN low AND high, without extra operators.

2. Step 2: Check each option

   SELECT * FROM A JOIN B ON A.value BETWEEN B.min AND B.max; uses correct syntax: A.value BETWEEN B.min AND B.max. Others misuse BETWEEN or add extra operators.

3. Final Answer:

   SELECT * FROM A JOIN B ON A.value BETWEEN B.min AND B.max; -> Option D

4. Quick Check:

   BETWEEN syntax = column BETWEEN low AND high [OK]

Hint: BETWEEN syntax: column BETWEEN low AND high, no extra operators [OK]

Common Mistakes:

• Adding = before BETWEEN
• Using IN BETWEEN instead of BETWEEN
• Placing BETWEEN incorrectly in ON clause

3. Given tables Products(product_id, price) and Discounts(min_price, max_price, discount_rate), what does this query return?

SELECT p.product_id, d.discount_rate
FROM Products p
JOIN Discounts d ON p.price >= d.min_price AND p.price < d.max_price;

medium

A. Only products with price exactly equal to min_price or max_price.

B. All products joined with all discounts regardless of price.

C. All products with their matching discount rate based on price ranges.

D. Syntax error due to invalid join condition.

Solution

1. Step 1: Analyze join condition

   The join matches products where price is between min_price (inclusive) and max_price (exclusive).

2. Step 2: Understand result

   This returns products with their discount rate if their price falls in the discount's price range.

3. Final Answer:

   All products with their matching discount rate based on price ranges. -> Option C

4. Quick Check:

   Non-equi join matches price ranges = All products with their matching discount rate based on price ranges. [OK]

Hint: Non-equi join matches ranges using >= and < [OK]

Common Mistakes:

• Thinking only exact matches are returned
• Assuming all products join with all discounts
• Believing the query has syntax errors

4. Identify the error in this non-equi join query:

SELECT e.name, s.salary_grade
FROM Employees e
JOIN SalaryGrades s ON e.salary => s.min_salary AND e.salary <= s.max_salary;

medium

A. The join condition should use OR instead of AND.

B. The operator => is invalid; it should be >=.

C. The table alias 's' is missing in the SELECT clause.

D. The query is missing a WHERE clause.

Solution

1. Step 1: Check operators in join condition

   The operator => is not valid SQL; the correct operator for 'greater than or equal' is >=.

2. Step 2: Verify other parts

   AND is correct to check salary between min and max. Aliases and WHERE clause are not errors here.

3. Final Answer:

   The operator => is invalid; it should be >=. -> Option B

4. Quick Check:

   Use >=, not => for greater or equal [OK]

Hint: Use >=, not =>, for greater or equal operator [OK]

Common Mistakes:

• Typing => instead of >=
• Replacing AND with OR incorrectly
• Confusing alias usage in SELECT

5. You have a table Scores(student_id, score) and a table Grades(grade, min_score, max_score). Write a query to assign each student their grade based on their score using a non-equi join.
Which query correctly implements this?

hard

A. SELECT s.student_id, g.grade FROM Scores s JOIN Grades g ON s.score >= g.min_score AND s.score < g.max_score;

B. SELECT s.student_id, g.grade FROM Scores s JOIN Grades g ON s.score <= g.min_score AND s.score >= g.max_score;

C. SELECT s.student_id, g.grade FROM Scores s JOIN Grades g ON s.score > g.min_score AND s.score <= g.max_score;

D. SELECT s.student_id, g.grade FROM Scores s JOIN Grades g ON s.score BETWEEN g.min_score AND g.max_score;

Solution

1. Step 1: Understand grading ranges

   Grades are assigned where score is between min_score (inclusive) and max_score (exclusive) to avoid overlap.

2. Step 2: Check each join condition

   SELECT s.student_id, g.grade FROM Scores s JOIN Grades g ON s.score >= g.min_score AND s.score < g.max_score; uses s.score >= g.min_score AND s.score < g.max_score, correctly defining non-overlapping ranges.

3. Step 3: Verify other options

   SELECT s.student_id, g.grade FROM Scores s JOIN Grades g ON s.score BETWEEN g.min_score AND g.max_score; includes max_score in BETWEEN (inclusive), which may cause overlap. SELECT s.student_id, g.grade FROM Scores s JOIN Grades g ON s.score > g.min_score AND s.score <= g.max_score; reverses inclusivity. SELECT s.student_id, g.grade FROM Scores s JOIN Grades g ON s.score <= g.min_score AND s.score >= g.max_score; reverses logic incorrectly.

4. Final Answer:

   SELECT s.student_id, g.grade FROM Scores s JOIN Grades g ON s.score >= g.min_score AND s.score < g.max_score; -> Option A

5. Quick Check:

   Use >= min and < max for non-overlapping ranges [OK]

Hint: Use >= min_score and < max_score for clean grade ranges [OK]

Common Mistakes:

• Using BETWEEN which includes max_score causing overlap
• Swapping < and > operators
• Using incorrect inclusivity causing duplicate grades