INTERSECT for common rows in SQL - Step-by-Step Execution

Concept Flow - INTERSECT for common rows

Start with Query1 Result Set

↓

Start with Query2 Result Set

↓

Compare Rows from Both Sets

↓

Keep Only Rows Present in Both

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Return Common Rows as Result

The INTERSECT operation compares two query results and returns only the rows that appear in both.

Execution Sample

SQL

SELECT name FROM Employees
INTERSECT
SELECT name FROM Managers;

This query returns the names that appear in both Employees and Managers tables.

Execution Table

Step	Query1 Result	Query2 Result	Comparison	Output Rows
1	Alice, Bob, Carol, Dave	Bob, Carol, Eve	Compare each name	Bob, Carol
2	No more rows to compare	No more rows to compare	Finished comparing	Final output: Bob, Carol

💡 All rows compared; only common rows Bob and Carol remain.

Variable Tracker

Variable	Start	After Step 1	Final
Query1 Result	Empty	Alice, Bob, Carol, Dave	Alice, Bob, Carol, Dave
Query2 Result	Empty	Bob, Carol, Eve	Bob, Carol, Eve
Output Rows	Empty	Bob, Carol	Bob, Carol

Key Moments - 2 Insights

Why does the output only include 'Bob' and 'Carol'?

Because INTERSECT returns only rows that appear in both Query1 and Query2 results, as shown in execution_table row 1.

What happens if a name appears only in one query result?

It is excluded from the output since INTERSECT keeps only common rows, demonstrated by 'Alice', 'Dave', and 'Eve' not appearing in the output.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 1?

A Alice, Bob

B Carol, Eve

C Bob, Carol

D Alice, Dave

💡 Hint

Check the 'Output Rows' column in execution_table row 1.

At which step does the comparison finish?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Step' column and 'Comparison' description in execution_table.

If 'Eve' was also in Employees, how would the output change?

A Output would include Eve

B Output would exclude Eve

C Output would be empty

D Output would include only Eve

💡 Hint

INTERSECT returns rows common to both queries; see variable_tracker for output changes.

Concept Snapshot

INTERSECT returns rows common to two queries.
Syntax: SELECT columns FROM table1
INTERSECT
SELECT columns FROM table2;
Only rows in both results appear.
Duplicates removed automatically.
Useful to find shared data.

Full Transcript

The INTERSECT operation in SQL compares two query results and returns only the rows that appear in both. For example, if we select names from Employees and Managers tables, INTERSECT returns only the names present in both. The execution table shows the first query returns Alice, Bob, Carol, Dave; the second returns Bob, Carol, Eve. Comparing these, only Bob and Carol appear in both, so they are the output. Rows unique to one query are excluded. This process finishes after all rows are compared. If a name like Eve was also in Employees, it would appear in the output because it is common to both. INTERSECT is useful to find shared data between two sets.

Practice

1. What does the SQL INTERSECT operator do?

easy

A. Combines rows from both queries including duplicates.

B. Returns all rows from the first SELECT query.

C. Returns all rows from the second SELECT query.

D. Returns only the rows common to both SELECT queries.

Solution

1. Step 1: Understand the purpose of INTERSECT

   The INTERSECT operator compares two SELECT queries and returns only the rows that appear in both results.

2. Step 2: Compare with other set operators

   Unlike UNION or UNION ALL, INTERSECT excludes rows not common to both queries.

3. Final Answer:

   Returns only the rows common to both SELECT queries. -> Option D

4. Quick Check:

   INTERSECT = common rows [OK]

Hint: INTERSECT keeps only shared rows between two queries [OK]

Common Mistakes:

• Confusing INTERSECT with UNION which combines all rows
• Thinking INTERSECT returns rows from only one query
• Assuming INTERSECT includes duplicates

2. Which of the following is the correct syntax to find common rows between two tables TableA and TableB with the same columns id and name?

easy

A. SELECT id, name FROM TableA JOIN TableB ON TableA.id = TableB.id;

B. SELECT id, name FROM TableA UNION SELECT id, name FROM TableB;

C. SELECT id, name FROM TableA INTERSECT SELECT id, name FROM TableB;

D. SELECT id, name FROM TableA WHERE id IN TableB;

Solution

1. Step 1: Identify correct INTERSECT syntax

   The INTERSECT operator is used between two SELECT statements with matching columns and types.

2. Step 2: Check each option

   SELECT id, name FROM TableA INTERSECT SELECT id, name FROM TableB; uses INTERSECT correctly. SELECT id, name FROM TableA UNION SELECT id, name FROM TableB; uses UNION which combines rows. SELECT id, name FROM TableA JOIN TableB ON TableA.id = TableB.id; uses JOIN which is different. SELECT id, name FROM TableA WHERE id IN TableB; has incorrect WHERE syntax.

3. Final Answer:

   SELECT id, name FROM TableA INTERSECT SELECT id, name FROM TableB; -> Option C

4. Quick Check:

   Correct INTERSECT syntax = SELECT id, name FROM TableA INTERSECT SELECT id, name FROM TableB; [OK]

Hint: Use INTERSECT between two SELECTs with matching columns [OK]

Common Mistakes:

• Using UNION instead of INTERSECT
• Trying to use WHERE with IN incorrectly
• Confusing JOIN with INTERSECT

3. Given two tables:
Employees1:
id | name
1 | Alice
2 | Bob
3 | Carol

Employees2:
id | name
2 | Bob
3 | Carol
4 | Dave

What is the result of this query?

SELECT id, name FROM Employees1 INTERSECT SELECT id, name FROM Employees2;

medium

A. Rows with Bob and Carol only

B. Rows with Alice, Bob, Carol, and Dave

C. Rows with Alice only

D. Rows with Dave only

Solution

1. Step 1: List rows from both tables

   Employees1 has (1, Alice), (2, Bob), (3, Carol). Employees2 has (2, Bob), (3, Carol), (4, Dave).

2. Step 2: Find common rows

   Common rows are those present in both: (2, Bob) and (3, Carol).

3. Final Answer:

   Rows with Bob and Carol only -> Option A

4. Quick Check:

   INTERSECT returns common rows = Bob, Carol [OK]

Hint: INTERSECT returns only rows present in both tables [OK]

Common Mistakes:

• Including rows unique to one table
• Confusing UNION with INTERSECT
• Ignoring column matching in rows

4. Consider this SQL query:

SELECT id, name FROM Customers INTERSECT SELECT id, name FROM Orders;

It returns an error. What is the most likely cause?

medium

A. The columns selected have different data types in Customers and Orders.

B. INTERSECT cannot be used with SELECT statements.

C. The tables Customers and Orders do not exist.

D. INTERSECT requires a WHERE clause.

Solution

1. Step 1: Understand INTERSECT requirements

   INTERSECT requires both SELECT queries to have the same number of columns with compatible data types.

2. Step 2: Analyze error causes

   If columns differ in type (e.g., id is integer in one table and string in another), the query errors. Other options are incorrect because INTERSECT is valid syntax, tables must exist to run, and WHERE is optional.

3. Final Answer:

   The columns selected have different data types in Customers and Orders. -> Option A

4. Quick Check:

   Matching column types required for INTERSECT [OK]

Hint: Check column types match for INTERSECT queries [OK]

Common Mistakes:

• Assuming INTERSECT needs WHERE clause
• Thinking INTERSECT is invalid syntax
• Ignoring data type mismatches

5. You have two tables:
ProductsA with columns product_id, name, price
ProductsB with columns product_id, name, price

You want to find products that exist in both tables with the same product_id and name, ignoring price differences.

Which query correctly uses INTERSECT to achieve this?

hard

A. SELECT product_id, name, price FROM ProductsA INTERSECT SELECT product_id, name, price FROM ProductsB;

B. SELECT product_id, name FROM ProductsA INTERSECT SELECT product_id, name FROM ProductsB;

C. SELECT product_id FROM ProductsA INTERSECT SELECT product_id FROM ProductsB;

D. SELECT * FROM ProductsA INTERSECT SELECT * FROM ProductsB;

Solution

1. Step 1: Identify columns to compare

   You want to compare only product_id and name, ignoring price differences.

2. Step 2: Use INTERSECT on matching columns

   SELECT product_id, name FROM ProductsA INTERSECT SELECT product_id, name FROM ProductsB; selects product_id and name from both tables and intersects them, returning only products common by those two columns.

3. Step 3: Analyze other options

   SELECT product_id, name, price FROM ProductsA INTERSECT SELECT product_id, name, price FROM ProductsB; includes price, so products with different prices won't match. SELECT product_id FROM ProductsA INTERSECT SELECT product_id FROM ProductsB; compares only product_id, ignoring name. SELECT * FROM ProductsA INTERSECT SELECT * FROM ProductsB; compares all columns, including price, which is not desired.

4. Final Answer:

   SELECT product_id, name FROM ProductsA INTERSECT SELECT product_id, name FROM ProductsB; -> Option B

5. Quick Check:

   INTERSECT on selected columns matches desired fields [OK]

Hint: Select only columns to compare before INTERSECT [OK]

Common Mistakes:

• Including extra columns that cause mismatches
• Selecting * when columns differ
• Comparing only one column when two are needed