REPLACE INTO behavior in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the REPLACE INTO statement do in MySQL?

It inserts a new row into a table. If a row with the same primary key or unique key already exists, it deletes that row first and then inserts the new row.

intermediate

How does REPLACE INTO differ from INSERT INTO ... ON DUPLICATE KEY UPDATE?

REPLACE INTO deletes the old row and inserts a new one, which can cause triggers and auto-increment values to behave differently. INSERT ... ON DUPLICATE KEY UPDATE updates the existing row without deleting it.

intermediate

What happens to auto-increment values when using REPLACE INTO?

Because REPLACE INTO deletes the old row and inserts a new one, the auto-increment counter increases as if a new row was inserted, even if replacing an existing row.

advanced

Can REPLACE INTO cause loss of data in some columns?

Yes. Since it deletes the old row and inserts a new one, any columns not specified in the REPLACE INTO statement will be set to their default values, potentially losing data.

beginner

When should you prefer REPLACE INTO over INSERT INTO?

Use REPLACE INTO when you want to ensure a row is inserted or replaced entirely based on unique keys, and you want to overwrite the entire row rather than update specific columns.

What does REPLACE INTO do if a row with the same primary key exists?

A Deletes the old row and inserts the new row

B Updates the existing row without deleting

C Ignores the new row

D Throws an error

REPLACE INTO deletes the existing row with the same key and inserts the new row.

Which statement is true about auto-increment values with REPLACE INTO?

A Auto-increment value stays the same

B Auto-increment value decreases

C Auto-increment value increases as if a new row was inserted

D Auto-increment value resets to zero

Because REPLACE INTO deletes and inserts a row, the auto-increment counter increases.

What happens to columns not specified in a REPLACE INTO statement?

A They are ignored

B They keep their old values

C They cause an error

D They are set to default values

Since the old row is deleted, unspecified columns get default values on insert.

Which is a key difference between REPLACE INTO and INSERT ... ON DUPLICATE KEY UPDATE?

A <code>REPLACE INTO</code> deletes and reinserts rows

B <code>REPLACE INTO</code> updates rows without deleting

C <code>INSERT ... ON DUPLICATE KEY UPDATE</code> deletes rows

D Both behave exactly the same

REPLACE INTO deletes the old row and inserts a new one, unlike INSERT ... ON DUPLICATE KEY UPDATE.

When is it best to use REPLACE INTO?

A When you want to update only some columns

B When you want to insert or fully replace a row

C When you want to ignore duplicates

D When you want to delete rows

REPLACE INTO is used to insert or fully replace a row based on unique keys.

Explain how REPLACE INTO works in MySQL and what happens to existing rows with the same key.

Think about what happens if a duplicate key exists.

Describe the differences between REPLACE INTO and INSERT ... ON DUPLICATE KEY UPDATE.

Focus on how rows are handled differently.

Practice

1. What does the REPLACE INTO statement do in MySQL?

easy

A. Inserts a new row or replaces an existing row with the same unique key.

B. Only inserts a new row and never updates existing rows.

C. Deletes all rows in the table before inserting new data.

D. Updates all rows in the table without inserting new ones.

Solution

1. Step 1: Understand the purpose of REPLACE INTO

   The REPLACE INTO statement either inserts a new row or replaces an existing row if a unique key conflict occurs.

2. Step 2: Compare with other statements

   Unlike INSERT, it can replace rows, and unlike DELETE, it does not remove all rows before inserting.

3. Final Answer:

   Inserts a new row or replaces an existing row with the same unique key. -> Option A

4. Quick Check:

   REPLACE INTO = Insert or replace [OK]

Hint: REPLACE INTO inserts or updates by unique key conflict [OK]

Common Mistakes:

• Thinking REPLACE INTO only inserts without replacing
• Confusing REPLACE INTO with DELETE
• Assuming REPLACE INTO updates without deleting first

2. Which of the following is the correct syntax for using REPLACE INTO in MySQL?

easy

A. REPLACE INTO table_name (col1, col2) VALUES (val1, val2);

B. REPLACE table_name SET col1=val1, col2=val2;

C. REPLACE INTO table_name VALUES col1=val1, col2=val2;

D. REPLACE INTO table_name (col1, col2) SET (val1, val2);

Solution

1. Step 1: Recall REPLACE INTO syntax

   The correct syntax uses REPLACE INTO table_name (columns) VALUES (values); format.

2. Step 2: Identify incorrect options

   Options A, C, and D use invalid syntax like missing INTO, wrong SET usage, or wrong VALUES format.

3. Final Answer:

   REPLACE INTO table_name (col1, col2) VALUES (val1, val2); -> Option A

4. Quick Check:

   Correct syntax = REPLACE INTO table_name (col1, col2) VALUES (val1, val2); [OK]

Hint: Use REPLACE INTO table (cols) VALUES (vals) syntax [OK]

Common Mistakes:

• Omitting INTO keyword
• Using SET instead of VALUES
• Incorrect parentheses placement

3. Given a table users(id INT PRIMARY KEY, name VARCHAR(20)) with rows:
(1, 'Alice'), (2, 'Bob')
What will be the result after executing:
REPLACE INTO users VALUES (2, 'Charlie');
Then selecting all rows ordered by id?

medium

A. [(1, 'Alice')]

B. [(1, 'Alice'), (2, 'Bob'), (2, 'Charlie')]

C. [(2, 'Charlie')]

D. [(1, 'Alice'), (2, 'Charlie')]

Solution

1. Step 1: Understand REPLACE INTO effect on existing row

   Since id=2 exists, REPLACE deletes that row and inserts the new one with 'Charlie'.

2. Step 2: Check final table rows

   Row with id=1 remains unchanged, row with id=2 is replaced by (2, 'Charlie').

3. Final Answer:

   [(1, 'Alice'), (2, 'Charlie')] -> Option D

4. Quick Check:

   Row with id=2 replaced = [(1, 'Alice'), (2, 'Charlie')] [OK]

Hint: REPLACE deletes existing row with same key before insert [OK]

Common Mistakes:

• Expecting duplicate keys after REPLACE
• Thinking REPLACE only updates without deleting
• Assuming original row remains unchanged

4. You run this query:
REPLACE INTO products (id, name) VALUES (1, 'Pen');
But get an error: ERROR 1146 (42S02): Table 'products' doesn't exist
What is the likely cause?

medium

A. The id column is not a unique or primary key.

B. You forgot to specify the VALUES keyword.

C. The table products does not exist.

D. You used REPLACE INTO instead of INSERT INTO.

Solution

1. Step 1: Check syntax correctness

   The query syntax is correct with REPLACE INTO products (id, name) VALUES (1, 'Pen');.

2. Step 2: Identify cause of table error

   Error 1146 indicates the table products does not exist or is misspelled.

3. Final Answer:

   The table products does not exist. -> Option C

4. Quick Check:

   Table doesn't exist error (1146) means the table is missing [OK]

Hint: Syntax error often means missing or misspelled table [OK]

Common Mistakes:

• Assuming table error means wrong REPLACE INTO usage
• Ignoring table existence
• Confusing table error with syntax error

5. You have a table inventory(product_id INT PRIMARY KEY, quantity INT) with rows:
(101, 5), (102, 10)
You want to add 3 more units to product 101 or insert it if missing.
Which query correctly uses REPLACE INTO to do this?

hard

A. REPLACE INTO inventory VALUES (101, quantity + 3);

B. REPLACE INTO inventory VALUES (101, 8);

C. REPLACE INTO inventory (product_id, quantity) VALUES (101, (SELECT quantity + 3, 0 FROM inventory WHERE product_id=101));

D. REPLACE INTO inventory (product_id, quantity) VALUES (101, 3) ON DUPLICATE KEY UPDATE quantity = quantity + 3;

Solution

1. Step 1: Understand REPLACE INTO behavior

   REPLACE deletes existing row with product_id=101 and inserts new row with given quantity.

2. Step 2: Check options for correctness

   REPLACE INTO inventory VALUES (101, 8); replaces quantity with 8 (5+3). REPLACE INTO inventory VALUES (101, quantity + 3); is invalid syntax. REPLACE INTO inventory (product_id, quantity) VALUES (101, (SELECT quantity + 3, 0 FROM inventory WHERE product_id=101)); uses subquery in VALUES which is not allowed. REPLACE INTO inventory (product_id, quantity) VALUES (101, 3) ON DUPLICATE KEY UPDATE quantity = quantity + 3; mixes REPLACE and ON DUPLICATE KEY UPDATE which is invalid.

3. Final Answer:

   REPLACE INTO inventory VALUES (101, 8); -> Option B

4. Quick Check:

   REPLACE needs full new row, no expressions [OK]

Hint: REPLACE INTO needs full values, no expressions or subqueries [OK]

Common Mistakes:

• Using expressions like quantity + 3 directly in VALUES
• Mixing REPLACE INTO with ON DUPLICATE KEY UPDATE
• Using subqueries inside VALUES clause