Insert, update, delete operations in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the INSERT operation in PHP with databases?

The INSERT operation adds new records (rows) into a database table.

beginner

How do you update a record in a database using PHP?

You use the UPDATE SQL statement with PHP to change existing data in a table, usually specifying which record to update with a WHERE clause.

beginner

What does the DELETE operation do in PHP database handling?

DELETE removes one or more records from a database table based on a condition.

intermediate

Show a simple PHP code snippet to insert a new user with name and email into a MySQL database.

$name = 'Alice'; $email = '[e-mail]'; $sql = "INSERT INTO users (name, email) VALUES ('$name', '$email')"; mysqli_query($conn, $sql);

beginner

Why is it important to use a WHERE clause in UPDATE and DELETE operations?

Without a WHERE clause, UPDATE or DELETE will affect all rows in the table, which can cause data loss or unwanted changes.

Which SQL statement is used to add new data to a table?

A INSERT

B UPDATE

C DELETE

D SELECT

INSERT adds new rows to a table.

What happens if you run an UPDATE statement without a WHERE clause?

A No rows are updated

B Only one row is updated

C All rows in the table are updated

D The table is deleted

Without WHERE, UPDATE affects every row.

Which PHP function is commonly used to execute SQL queries?

A mysqli_query()

B execute_sql()

C run_query()

D sql_execute()

mysqli_query() runs SQL commands in PHP.

What does the DELETE statement do?

A Adds new records

B Changes existing records

C Retrieves records

D Removes records

DELETE removes records from a table.

Which clause specifies which rows to update or delete?

A FROM

B WHERE

C SET

D VALUES

WHERE filters rows for UPDATE or DELETE.

Explain how to safely insert, update, and delete records in a database using PHP.

Think about the SQL commands and how PHP runs them.

Describe the risks of running UPDATE or DELETE operations without a WHERE clause in PHP database code.

What happens if you forget to specify which rows to change?

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