Set operation column matching rules in SQL

Introduction

Set operations combine results from two or more queries. The columns must match so the database knows how to join the data.

You want to combine lists of customers from two different stores.

You need to merge product lists from two suppliers into one list.

You want to find common items sold in two different years.

You want to get all unique email addresses from two mailing lists.

Syntax

SQL

SELECT column1, column2, ... FROM table1
SET_OPERATION
SELECT column1, column2, ... FROM table2;

Both SELECT statements must have the same number of columns.

The data types of corresponding columns should be compatible.

Examples

This combines employee and contractor names and ages into one list without duplicates.

SQL

SELECT name, age FROM employees
UNION
SELECT name, age FROM contractors;

This finds products with the same ID and price in both stores.

SQL

SELECT product_id, price FROM store1
INTERSECT
SELECT product_id, price FROM store2;

This shows cities and states where customers are only in the north region.

SQL

SELECT city, state FROM customers_north
EXCEPT
SELECT city, state FROM customers_south;

Sample Program

This example creates two fruit tables and combines their lists without duplicates using UNION.

SQL

CREATE TABLE fruits1 (name VARCHAR(20), color VARCHAR(20));
INSERT INTO fruits1 VALUES ('Apple', 'Red'), ('Banana', 'Yellow');

CREATE TABLE fruits2 (name VARCHAR(20), color VARCHAR(20));
INSERT INTO fruits2 VALUES ('Banana', 'Yellow'), ('Grape', 'Purple');

SELECT name, color FROM fruits1
UNION
SELECT name, color FROM fruits2;

Important Notes

Column names in the final result come from the first SELECT statement.

UNION removes duplicates, UNION ALL keeps duplicates.

Data types should be compatible but do not need to be exactly the same.

Summary

Set operations combine results from multiple queries.

Columns must match in number and compatible types.

Column names come from the first query in the set operation.

Practice

1.

Which rule must be followed when using SQL set operations like UNION or INTERSECT?

easy

A. Both queries must have the same number of columns with compatible data types.

B. The second query must have more columns than the first.

C. Column names must be identical in both queries.

D. Set operations only work with numeric columns.

Solution

1. Step 1: Understand set operation requirements

   Set operations combine results from multiple queries, so columns must match in number and type to align data correctly.

2. Step 2: Check column name and type rules

   Column names do not need to match; only the first query's column names are used. Data types must be compatible, and the number of columns must be the same.

3. Final Answer:

   Both queries must have the same number of columns with compatible data types. -> Option A

4. Quick Check:

   Column count and type compatibility = A [OK]

Hint: Match column count and types for set operations [OK]

Common Mistakes:

• Assuming column names must match
• Using different number of columns
• Trying set operations on incompatible data types

2.

Which of the following SQL queries correctly uses UNION with matching columns?

-- Table A: (id INT, name VARCHAR)
-- Table B: (user_id INT, username VARCHAR)

A) SELECT id, name FROM A UNION SELECT user_id, username FROM B;
B) SELECT id FROM A UNION SELECT user_id, username FROM B;
C) SELECT id, name FROM A UNION SELECT user_id FROM B;
D) SELECT id, name FROM A UNION SELECT user_id, username, email FROM B;

easy

A. SELECT id, name FROM A UNION SELECT user_id FROM B;

B. SELECT id FROM A UNION SELECT user_id, username FROM B;

C. SELECT id, name FROM A UNION SELECT user_id, username FROM B;

D. SELECT id, name FROM A UNION SELECT user_id, username, email FROM B;

Solution

1. Step 1: Check column counts in each query

   SELECT id, name FROM A UNION SELECT user_id, username FROM B; selects 2 columns from both queries, matching counts. Other options have mismatched column counts (1 vs 2, 2 vs 1, or 2 vs 3).

2. Step 2: Verify column types compatibility

   Columns in SELECT id, name FROM A UNION SELECT user_id, username FROM B; are INT and VARCHAR in both queries, which are compatible.

3. Final Answer:

   SELECT id, name FROM A UNION SELECT user_id, username FROM B; -> Option C

4. Quick Check:

   Equal columns and compatible types = A [OK]

Hint: Count columns and check types match in both queries [OK]

Common Mistakes:

• Mismatched column counts cause errors
• Ignoring column type compatibility
• Assuming extra columns are allowed

3.

Given the tables and query below, what will be the output?

Table X:
id | value
1  | 'A'
2  | 'B'

Table Y:
code | val
2    | 'B'
3    | 'C'

Query:
SELECT id, value FROM X
UNION
SELECT code, val FROM Y;

medium

A. Syntax error due to column name mismatch

B. [ (1, 'A'), (2, 'B'), (3, 'C') ]

C. [ (1, 'A'), (3, 'C') ]

D. [ (1, 'A'), (2, 'B'), (2, 'B'), (3, 'C') ]

Solution

1. Step 1: Check column counts and types

   Both queries select 2 columns with compatible types (INT and VARCHAR). Column names differ but that is allowed.

2. Step 2: Understand UNION behavior

   UNION removes duplicates. Rows (2, 'B') appear in both tables, so only one copy appears in result.

3. Final Answer:

   [ (1, 'A'), (2, 'B'), (3, 'C') ] -> Option B

4. Quick Check:

   UNION removes duplicates = D [OK]

Hint: UNION removes duplicates, column names don't matter [OK]

Common Mistakes:

• Expecting duplicate rows in UNION result
• Thinking column names must match
• Confusing UNION with UNION ALL

4.

Identify the error in the following SQL set operation:

SELECT id, name FROM Customers
UNION
SELECT customer_id FROM Orders;

medium

A. Column names do not match between queries.

B. UNION cannot be used with SELECT statements.

C. Data types of columns are incompatible.

D. The second query has fewer columns than the first.

Solution

1. Step 1: Compare column counts in both queries

   The first query selects 2 columns (id, name), the second selects only 1 column (customer_id). This mismatch causes an error.

2. Step 2: Check other possible errors

   Column names do not need to match, and UNION works with SELECT statements. Data types are unknown but column count mismatch is enough to cause error.

3. Final Answer:

   The second query has fewer columns than the first. -> Option D

4. Quick Check:

   Column count mismatch = B [OK]

Hint: Ensure both queries select same number of columns [OK]

Common Mistakes:

• Thinking column names must match
• Ignoring column count mismatch
• Assuming UNION works with different column counts

5.

You have two tables:

Employees(emp_id INT, emp_name VARCHAR, dept VARCHAR)
Managers(manager_id INT, manager_name VARCHAR)

You want to combine employee and manager names into one list using a set operation. Which query correctly applies set operation column matching rules?

A) SELECT emp_id, emp_name FROM Employees UNION SELECT manager_id, manager_name FROM Managers;
B) SELECT emp_name FROM Employees UNION SELECT manager_name FROM Managers;
C) SELECT emp_name, dept FROM Employees UNION SELECT manager_name, manager_id FROM Managers;
D) SELECT emp_id, emp_name, dept FROM Employees UNION SELECT manager_id, manager_name FROM Managers;

hard

A. SELECT emp_name FROM Employees UNION SELECT manager_name FROM Managers;

B. SELECT emp_id, emp_name FROM Employees UNION SELECT manager_id, manager_name FROM Managers;

C. SELECT emp_name, dept FROM Employees UNION SELECT manager_name, manager_id FROM Managers;

D. SELECT emp_id, emp_name, dept FROM Employees UNION SELECT manager_id, manager_name FROM Managers;

Solution

1. Step 1: Identify columns to combine

   You want a list of names only, so selecting one column (name) from each table is appropriate.

2. Step 2: Check column counts and types

   SELECT emp_name FROM Employees UNION SELECT manager_name FROM Managers; selects one VARCHAR column from each table, matching count and compatible types. Other options have mismatched column counts or incompatible columns.

3. Final Answer:

   SELECT emp_name FROM Employees UNION SELECT manager_name FROM Managers; -> Option A

4. Quick Check:

   Same column count and type for names = B [OK]

Hint: Select same number and type of columns to combine [OK]

Common Mistakes:

• Selecting different numbers of columns
• Mixing incompatible column types
• Including unrelated columns in set operation