Insert, update, delete operations in PHP - Step-by-Step Execution

Concept Flow - Insert, update, delete operations

Start

↓

Choose Operation

↓

Insert

↓

Execute SQL Query

↓

Check Success?

No→Show Error

Yes↓

Finish

This flow shows choosing an operation (insert, update, delete), running the SQL query, checking if it worked, then finishing.

Execution Sample

PHP

<?php
// Insert
$sql = "INSERT INTO users (name, age) VALUES ('Alice', 30)";
// Update
$sql = "UPDATE users SET age = 31 WHERE name = 'Alice'";
// Delete
$sql = "DELETE FROM users WHERE name = 'Alice'";
?>

This code shows SQL commands for inserting, updating, and deleting a user named Alice.

Execution Table

Step	Operation	SQL Query	Execution Result	Action Taken
1	Insert	INSERT INTO users (name, age) VALUES ('Alice', 30)	Success	User Alice added
2	Update	UPDATE users SET age = 31 WHERE name = 'Alice'	Success	User Alice age updated to 31
3	Delete	DELETE FROM users WHERE name = 'Alice'	Success	User Alice removed
4	Insert	INSERT INTO users (name, age) VALUES ('Alice', 30)	Fail	Error shown: Duplicate entry
5	Update	UPDATE users SET age = 31 WHERE name = 'Bob'	Fail	Error shown: User not found
6	Delete	DELETE FROM users WHERE name = 'Bob'	Fail	Error shown: User not found

💡 Execution stops after all operations tried or on error handling.

Variable Tracker

Variable	Start	After Insert	After Update	After Delete
users table	empty	[{'name':'Alice','age':30}]	[{'name':'Alice','age':31}]	empty

Key Moments - 3 Insights

Why does the update operation only change Alice's age and not add a new user?

Because the SQL UPDATE command changes existing rows matching the WHERE condition. If no row matches, nothing is added. See execution_table row 2 where update succeeds only if user exists.

What happens if we try to insert a user that already exists?

The insert fails due to duplicate entry (primary key or unique constraint). See execution_table row 4 where insert fails and error is shown.

Why does delete fail when trying to remove a user not in the table?

Because DELETE affects only rows matching the WHERE condition. If none match, no rows deleted and often an error or no change occurs. See execution_table row 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the SQL query used in step 2?

A UPDATE users SET age = 31 WHERE name = 'Alice'

B INSERT INTO users (name, age) VALUES ('Alice', 30)

C DELETE FROM users WHERE name = 'Alice'

D UPDATE users SET age = 31 WHERE name = 'Bob'

💡 Hint

Check the 'SQL Query' column in execution_table row 2.

At which step does the users table become empty again?

A After Update

B After Delete

C After Insert

D Start

💡 Hint

Look at variable_tracker row for 'users table' after each operation.

If we try to update a user named 'Bob' who does not exist, what happens?

A User Bob is added

B Update succeeds silently

C Update fails and error is shown

D User Alice is updated instead

💡 Hint

See execution_table row 5 for update operation on non-existing user.

Concept Snapshot

Insert, update, delete are SQL commands to add, change, or remove data.
Insert adds new rows.
Update changes existing rows matching a condition.
Delete removes rows matching a condition.
Always check if operation succeeded to handle errors.

Full Transcript

This visual execution shows how insert, update, and delete operations work in PHP with SQL. First, you choose the operation, then run the SQL query. Insert adds a new user named Alice. Update changes Alice's age to 31. Delete removes Alice from the table. If you try to insert Alice again, it fails due to duplicate. Updating or deleting a user not in the table also fails. Variables track the users table state after each operation. Key moments explain why update doesn't add new users, why insert can fail on duplicates, and why delete fails if user not found. The quiz tests understanding of SQL queries, table state changes, and error cases. Remember to always check if your database operation succeeded.

Practice

1. Which SQL statement is used in PHP to add new data into a database table?

easy

A. UPDATE

B. INSERT INTO

C. DELETE FROM

D. SELECT

Solution

1. Step 1: Understand the purpose of each SQL command

   INSERT INTO adds new rows, UPDATE changes existing rows, DELETE FROM removes rows, and SELECT retrieves data.

2. Step 2: Match the command to adding new data

   Since adding new data means inserting new rows, the correct command is INSERT INTO.

3. Final Answer:

   INSERT INTO -> Option B

4. Quick Check:

   Insert = Add new data [OK]

Hint: Insert adds new rows; update and delete modify existing ones [OK]

Common Mistakes:

• Confusing UPDATE with INSERT
• Thinking DELETE adds data
• Using SELECT to add data

2. Which of the following is the correct PHP syntax to update a user's email in a MySQL database using mysqli_query?

easy

A. $sql = "DELETE FROM users SET email = '[e-mail]' WHERE id = 1";

B. $sql = "INSERT INTO users email = '[e-mail]' WHERE id = 1";

C. $sql = "SELECT email = '[e-mail]' FROM users WHERE id = 1";

D. $sql = "UPDATE users SET email = '[e-mail]' WHERE id = 1";

Solution

1. Step 1: Identify the correct SQL syntax for updating

   The correct syntax for updating is UPDATE table SET column = value WHERE condition.

2. Step 2: Check each option's syntax

   $sql = "UPDATE users SET email = '[e-mail]' WHERE id = 1"; matches the correct syntax. Options A, B, and D misuse SQL commands or syntax.

3. Final Answer:

   $sql = "UPDATE users SET email = '[e-mail]' WHERE id = 1"; -> Option D

4. Quick Check:

   Update syntax = UPDATE ... SET ... WHERE ... [OK]

Hint: Update uses SET with WHERE to change specific rows [OK]

Common Mistakes:

• Using INSERT syntax for update
• Omitting WHERE clause in update
• Using DELETE or SELECT instead of UPDATE

3. What will be the output of the following PHP code snippet?

$conn = new mysqli('localhost', 'user', 'pass', 'db');
$sql = "DELETE FROM products WHERE id = 10";
if ($conn->query($sql) === TRUE) {
    echo "Deleted successfully";
} else {
    echo "Error deleting record";
}

medium

A. No output

B. Error deleting record

C. Deleted successfully

D. Syntax error

Solution

1. Step 1: Understand the code logic

   The code tries to delete a product with id 10. If the query runs without error, it prints "Deleted successfully".

2. Step 2: Assume the query runs correctly

   Since the SQL syntax is correct and connection is assumed valid, the query returns TRUE, so the success message prints.

3. Final Answer:

   Deleted successfully -> Option C

4. Quick Check:

   Successful DELETE query prints success message [OK]

Hint: If query returns TRUE, success message prints [OK]

Common Mistakes:

• Assuming syntax error without checking code
• Confusing TRUE with FALSE return values
• Ignoring the if-else structure

4. Identify the error in this PHP code that tries to update a user's name:

$sql = "UPDATE users SET name = 'John'";
$conn->query($sql);

medium

A. Missing WHERE clause to specify which user to update

B. Incorrect SQL keyword, should be INSERT

C. Quotes around 'John' should be double quotes

D. Missing semicolon after query execution

Solution

1. Step 1: Review the SQL update statement

   The statement updates the name but does not specify which row to update because it lacks a WHERE clause.

2. Step 2: Understand the impact of missing WHERE

   Without WHERE, all rows in the table will be updated, which is usually unintended and considered an error.

3. Final Answer:

   Missing WHERE clause to specify which user to update -> Option A

4. Quick Check:

   Update without WHERE affects all rows [OK]

Hint: Always include WHERE in update to target specific rows [OK]

Common Mistakes:

• Thinking quotes type causes error
• Confusing UPDATE with INSERT
• Ignoring missing WHERE consequences

5. You want to delete all users from the users table who have not logged in for over a year. Which PHP code snippet correctly performs this operation assuming the last login date is stored in last_login column?

hard

A. $sql = "DELETE FROM users WHERE last_login < DATE_SUB(NOW(), INTERVAL 1 YEAR)";
$conn->query($sql);

B. $sql = "UPDATE users SET last_login = NULL WHERE last_login > DATE_SUB(NOW(), INTERVAL 1 YEAR)";
$conn->query($sql);

C. $sql = "DELETE FROM users WHERE last_login > DATE_ADD(NOW(), INTERVAL 1 YEAR)";
$conn->query($sql);

D. $sql = "INSERT INTO users (last_login) VALUES (DATE_SUB(NOW(), INTERVAL 1 YEAR))";
$conn->query($sql);

Solution

1. Step 1: Understand the condition for deletion

   We want to delete users whose last_login is older than one year, so the condition is last_login < DATE_SUB(NOW(), INTERVAL 1 YEAR).

2. Step 2: Check each option's SQL logic

   $sql = "DELETE FROM users WHERE last_login < DATE_SUB(NOW(), INTERVAL 1 YEAR)"; $conn->query($sql); correctly deletes users with last login older than one year. $sql = "UPDATE users SET last_login = NULL WHERE last_login > DATE_SUB(NOW(), INTERVAL 1 YEAR)"; $conn->query($sql); updates instead of deletes. $sql = "DELETE FROM users WHERE last_login > DATE_ADD(NOW(), INTERVAL 1 YEAR)"; $conn->query($sql); deletes users with last login in the future, which is wrong. $sql = "INSERT INTO users (last_login) VALUES (DATE_SUB(NOW(), INTERVAL 1 YEAR))"; $conn->query($sql); inserts new rows, which is incorrect.

3. Final Answer:

   $sql = "DELETE FROM users WHERE last_login < DATE_SUB(NOW(), INTERVAL 1 YEAR)"; $conn->query($sql); -> Option A

4. Quick Check:

   Delete with condition older than 1 year = $sql = "DELETE FROM users WHERE last_login < DATE_SUB(NOW(), INTERVAL 1 YEAR)"; $conn->query($sql); [OK]

Hint: Use DELETE with WHERE and DATE_SUB for time conditions [OK]

Common Mistakes:

• Using UPDATE instead of DELETE
• Wrong date comparison operators
• Using INSERT instead of DELETE