Common INSERT errors and fixes in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this INSERT query?

Consider a table users with columns id INT PRIMARY KEY and name VARCHAR(50). What happens when you run this query?

INSERT INTO users (id, name) VALUES (1, 'Alice'), (1, 'Bob');

SQL

INSERT INTO users (id, name) VALUES (1, 'Alice'), (1, 'Bob');

A Syntax error due to multiple rows in VALUES clause.

B Both rows are inserted successfully.

C Only the first row is inserted; the second causes a primary key violation error.

D The second row overwrites the first row silently.

💡 Hint

Primary keys must be unique. What happens if you insert duplicate keys?

The primary key column 'id' must have unique values. Trying to insert two rows with the same 'id' value 1 causes a primary key violation error on the second row.

❓ query_result

intermediate

What error does this INSERT cause?

Given a table products with columns product_id INT, name VARCHAR(100), and price DECIMAL(5,2), what error occurs when running:

INSERT INTO products (product_id, name, price) VALUES (101, 'Pen', 'free');

SQL

INSERT INTO products (product_id, name, price) VALUES (101, 'Pen', 'free');

A Data type mismatch error because 'free' is not a valid decimal.

B The row is inserted with NULL price.

C Syntax error due to quotes around 'free'.

D No error; 'free' is converted to 0.00.

💡 Hint

Check the data type of the price column and the value provided.

The price column expects a decimal number. The string 'free' cannot be converted to a decimal, causing a data type mismatch error.

📝 Syntax

advanced

Which option fixes the syntax error in this INSERT?

This query causes a syntax error:

INSERT INTO orders order_id, customer_id VALUES (1001, 2002);

Which option fixes it?

SQL

INSERT INTO orders order_id, customer_id VALUES (1001, 2002);

A INSERT INTO orders VALUES (order_id, customer_id) (1001, 2002);

B INSERT INTO orders VALUES order_id, customer_id (1001, 2002);

C INSERT orders INTO (order_id, customer_id) VALUES (1001, 2002);

D INSERT INTO orders (order_id, customer_id) VALUES (1001, 2002);

💡 Hint

Check the correct syntax for specifying columns in INSERT.

The column list must be enclosed in parentheses immediately after the table name.

❓ query_result

advanced

What happens if you omit columns in INSERT?

Given a table employees with columns id INT PRIMARY KEY, name VARCHAR(50), and department VARCHAR(50) DEFAULT 'Sales', what is the result of:

INSERT INTO employees (id, name) VALUES (10, 'John');

SQL

INSERT INTO employees (id, name) VALUES (10, 'John');

A Row inserted with department set to 'Sales'.

B Error because department column is missing.

C Row inserted with department set to NULL.

D Row inserted but department column is empty string.

💡 Hint

What happens when a column has a default value and is not specified in INSERT?

If a column has a default value and is omitted in the INSERT, the default value is used.

🔧 Debug

expert

Why does this INSERT fail with a foreign key error?

Tables:

• departments: dept_id INT PRIMARY KEY, dept_name VARCHAR(50)
• employees: emp_id INT PRIMARY KEY, name VARCHAR(50), dept_id INT FOREIGN KEY REFERENCES departments(dept_id)

Query:

INSERT INTO employees (emp_id, name, dept_id) VALUES (1, 'Anna', 5);

Assuming no department with dept_id = 5 exists, what happens?

SQL

INSERT INTO employees (emp_id, name, dept_id) VALUES (1, 'Anna', 5);

A Row inserted with dept_id set to NULL automatically.

B Foreign key constraint violation error occurs.

C Syntax error due to missing department.

D Row inserted successfully; foreign key is ignored.

💡 Hint

Foreign keys require referenced values to exist.

The foreign key constraint requires that dept_id in employees must exist in departments. Since dept_id 5 does not exist, the insert fails.

Practice

1. What is a common cause of an INSERT statement failing in SQL?

easy

A. Adding extra spaces between keywords

B. Using SELECT instead of INSERT

C. Missing a value for a column that does not allow NULLs

D. Writing the table name in lowercase

Solution

1. Step 1: Understand column constraints

   Columns that do not allow NULL values must have a value provided during insert.

2. Step 2: Identify missing values impact

   If a required column is missing a value, the insert will fail with an error.

3. Final Answer:

   Missing a value for a column that does not allow NULLs -> Option C

4. Quick Check:

   Required columns need values [OK]

Hint: Always provide values for NOT NULL columns [OK]

Common Mistakes:

• Ignoring NOT NULL constraints
• Assuming case sensitivity causes errors
• Confusing SELECT with INSERT

2. Which of the following INSERT statements is syntactically correct for a table users(id, name)?

easy

A. INSERT INTO users VALUES (1, 'Alice');

B. INSERT users INTO VALUES (1, 'Alice');

C. INSERT INTO users (1, 'Alice');

D. INSERT INTO users VALUES 1, 'Alice';

Solution

1. Step 1: Review correct INSERT syntax

   The correct syntax is: INSERT INTO table_name VALUES (value1, value2, ...);

2. Step 2: Check each option

   INSERT INTO users VALUES (1, 'Alice'); matches the correct syntax exactly; others have misplaced keywords or missing parentheses.

3. Final Answer:

   INSERT INTO users VALUES (1, 'Alice'); -> Option A

4. Quick Check:

   INSERT INTO table VALUES (...) [OK]

Hint: Remember: INSERT INTO table VALUES (values) [OK]

Common Mistakes:

• Misplacing keywords order
• Omitting parentheses around values
• Using wrong keyword sequence

3. Given the table products(id INT PRIMARY KEY, name VARCHAR(20)), what happens when you run:
INSERT INTO products VALUES (1, 'Pen');
INSERT INTO products VALUES (1, 'Pencil');

medium

A. Syntax error on second insert

B. Both rows are inserted successfully

C. Second insert overwrites the first row

D. Second insert fails due to duplicate primary key

Solution

1. Step 1: Understand primary key constraint

   Primary keys must be unique; duplicate values cause insert failure.

2. Step 2: Analyze the inserts

   The first insert adds id=1. The second tries to add id=1 again, causing a duplicate key error.

3. Final Answer:

   Second insert fails due to duplicate primary key -> Option D

4. Quick Check:

   Primary key duplicates cause insert errors [OK]

Hint: Primary keys must be unique; duplicates fail [OK]

Common Mistakes:

• Assuming duplicates overwrite existing rows
• Ignoring primary key uniqueness
• Expecting syntax error instead of constraint error

4. You run this query:
INSERT INTO employees (id, name, age) VALUES (5, 'John');
What is the error and how to fix it?

medium

A. Syntax error: missing value for age; add the missing value

B. No error; query runs fine

C. Wrong table name; change employees to employee

D. Age column must be removed from column list

Solution

1. Step 1: Check column-value count

   Columns listed are 3, but only 2 values provided; this causes an error.

2. Step 2: Fix by adding missing value

   Provide a value for age or remove it from the column list to match values count.

3. Final Answer:

   Syntax error: missing value for age; add the missing value -> Option A

4. Quick Check:

   Columns and values count must match [OK]

Hint: Match columns count with values count [OK]

Common Mistakes:

• Providing fewer values than columns
• Assuming missing values default automatically
• Ignoring syntax errors from mismatch

5. You want to insert multiple rows into orders(order_id INT PRIMARY KEY, product VARCHAR(50), quantity INT). Which statement correctly inserts two rows without errors?

hard

A. INSERT INTO orders VALUES (101, 'Book', 3); INSERT INTO orders VALUES (101, 'Pen', 5);

B. INSERT INTO orders VALUES (101, 'Book', 3), (102, 'Pen', 5);

C. INSERT INTO orders (order_id, product) VALUES (101, 'Book'), (102, 'Pen');

D. INSERT INTO orders VALUES (101, 'Book', 'three'), (102, 'Pen', 5);

Solution

1. Step 1: Check multiple row insert syntax

   INSERT INTO orders VALUES (101, 'Book', 3), (102, 'Pen', 5); uses correct syntax to insert multiple rows in one statement.

2. Step 2: Validate data types and uniqueness

   INSERT INTO orders VALUES (101, 'Book', 3), (102, 'Pen', 5); provides unique order_id and correct data types; others have duplicates, missing columns, or wrong types.

3. Final Answer:

   INSERT INTO orders VALUES (101, 'Book', 3), (102, 'Pen', 5); -> Option B

4. Quick Check:

   Multiple rows insert with unique keys and correct types [OK]

Hint: Use comma-separated rows with matching columns [OK]

Common Mistakes:

• Duplicate primary keys in multiple inserts
• Missing required columns in multi-row insert
• Wrong data types causing errors