Executing queries with query method in PHP - Step-by-Step Execution

Concept Flow - Executing queries with query method

Create DB connection

↓

Prepare SQL query string

↓

Call query() method on connection

↓

Check if query succeeded

↓

Fetch results

↓

Use or display results

↓

Close connection

This flow shows how to run a SQL query using the query() method on a database connection, check success, then fetch or handle errors.

Execution Sample

PHP

<?php
$conn = new mysqli('localhost', 'user', 'pass', 'db');
$result = $conn->query('SELECT * FROM users');
if ($result) {
  while ($row = $result->fetch_assoc()) {
    echo $row['name'];
  }
}
$conn->close();
?>

This code connects to a MySQL database, runs a SELECT query using query(), then prints each user's name.

Execution Table

Step	Action	Evaluation	Result
1	Create mysqli connection	Connection established	Connection object created
2	Call query() with 'SELECT * FROM users'	Query sent to DB	Result object returned
3	Check if result is truthy	Result is valid	Enter fetch loop
4	Fetch first row with fetch_assoc()	Row data: ['name' => 'Alice']	Row array returned
5	Echo $row['name']	Output 'Alice'	Printed 'Alice'
6	Fetch second row	Row data: ['name' => 'Bob']	Row array returned
7	Echo $row['name']	Output 'Bob'	Printed 'Bob'
8	Fetch next row	No more rows	fetch_assoc() returns null
9	Exit loop	No more rows	Stop fetching
10	Close connection	Connection closed	Resources freed

💡 fetch_assoc() returns null when no more rows, ending the loop

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8	Final
$conn	null	mysqli connection object	mysqli connection object	mysqli connection object	mysqli connection object	closed
$result	null	mysqli_result object	mysqli_result object	mysqli_result object	mysqli_result object	mysqli_result object
$row	null	null	['name' => 'Alice']	['name' => 'Bob']	null	null

Key Moments - 3 Insights

Why do we check if $result is truthy before fetching rows?

Because query() returns false if the query fails. Checking $result ensures we only fetch rows if the query succeeded (see step 3 in execution_table).

What happens when fetch_assoc() returns null?

It means there are no more rows to fetch, so the loop ends (see step 8 and 9 in execution_table).

Why do we close the connection at the end?

Closing frees resources and ends the connection cleanly (step 10).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $row after step 6?

A ['name' => 'Bob']

B ['name' => 'Alice']

C null

D mysqli_result object

💡 Hint

Check the variable_tracker row for $row at After Step 6

At which step does the fetch loop end because there are no more rows?

A Step 7

B Step 9

C Step 8

D Step 10

💡 Hint

Look at execution_table rows where fetch_assoc() returns null

If the query failed, what would $result be after step 2?

A null

B false

C mysqli_result object

D empty array

💡 Hint

Recall that query() returns false on failure (see key_moments about checking $result)

Concept Snapshot

Use $conn->query($sql) to run SQL queries.
Check if result is truthy before fetching.
Use fetch_assoc() to get rows one by one.
fetch_assoc() returns null when done.
Close connection when finished.

Full Transcript

This example shows how to execute SQL queries in PHP using the query() method of a mysqli connection. First, a connection is created. Then, the query() method sends the SQL string to the database. If the query succeeds, it returns a result object. We check if the result is valid before fetching rows. Using fetch_assoc(), we get each row as an associative array until no rows remain. Each row's data can be used or printed. Finally, the connection is closed to free resources.