Set operation column matching rules in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all unique names from two tables using UNION.

SQL

SELECT name FROM employees UNION SELECT [1] FROM managers;

Drag options to blanks, or click blank then click option'

Aname

Bsalary

Cid

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting different columns in each SELECT statement causes errors.

Using columns with incompatible data types.

The UNION operation requires the columns to match by position and type. Here, both SELECT statements must select the 'name' column.

2fill in blank

medium

Complete the code to combine salaries from two tables using UNION ALL.

SQL

SELECT salary FROM full_time UNION ALL SELECT [1] FROM part_time;

Drag options to blanks, or click blank then click option'

Ahours

Bname

Csalary

Dposition

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting different columns causes errors.

Using UNION instead of UNION ALL when duplicates are needed.

UNION ALL combines all rows including duplicates. Both SELECT statements must select the 'salary' column to match.

3fill in blank

hard

Fix the error in the set operation by choosing the correct column to match.

SQL

SELECT id, name FROM customers INTERSECT SELECT [1], name FROM orders;

Drag options to blanks, or click blank then click option'

Aname

Bid

Ccustomer_id

Dorder_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using columns with different data types or meanings.

Mismatching the number of columns in SELECT statements.

Both SELECT statements must have the same number of columns and compatible types in the same order. Here, 'id' matches the first column.

4fill in blank

hard

Fill both blanks to correctly combine product names and prices from two tables using UNION.

SQL

SELECT [1], [2] FROM products_a UNION SELECT product_name, price FROM products_b;

Drag options to blanks, or click blank then click option'

Aproduct_name

Bprice

Cname

Dcost

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting columns with incompatible data types.

Mismatching the order of columns.

The first SELECT must select columns that match the second SELECT by position and type. 'name' matches 'product_name' in type, and 'price' matches 'price'.

5fill in blank

hard

Fill all three blanks to correctly perform a UNION of employee details with matching columns.

SQL

SELECT [1], [2], [3] FROM staff UNION SELECT emp_name, emp_salary, emp_dept FROM employees;

Drag options to blanks, or click blank then click option'

Aname

Bsalary

Cdepartment

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using columns with different meanings or data types.

Mismatching the number of columns.

Each column in the first SELECT must match the corresponding column in the second SELECT by position and type. Here, 'name' matches 'emp_name', 'salary' matches 'emp_salary', and 'department' matches 'emp_dept'.

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