Fetch modes and styles in PHP - Time & Space Complexity

Time Complexity: Fetch modes and styles

O(n)

Understanding Time Complexity

When fetching data from a database in PHP, the way we get results affects how long it takes.

We want to know how the time to fetch data grows as the amount of data increases.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// Fetch all rows as associative arrays
$stmt = $pdo->query('SELECT * FROM users');
$results = [];
while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
    $results[] = $row;
}

This code fetches all rows from a users table one by one as associative arrays and stores them in an array.

Identify Repeating Operations

• Primary operation: The while loop fetching each row from the database.
• How many times: Once for each row in the result set (n times).

How Execution Grows With Input

Each row requires one fetch operation, so the total work grows as the number of rows grows.

Input Size (n)	Approx. Operations
10	10 fetch calls
100	100 fetch calls
1000	1000 fetch calls

Pattern observation: The number of operations grows directly with the number of rows.

Final Time Complexity

Time Complexity: O(n)

This means the time to fetch all rows grows linearly with the number of rows.

Common Mistake

[X] Wrong: "Fetching all rows at once is always faster than fetching one by one."

[OK] Correct: Fetching all at once may use more memory and sometimes the fetch loop is just as efficient because it processes rows as they come.

Interview Connect

Understanding how fetching data scales helps you write efficient database code and shows you know how to handle growing data smoothly.

Self-Check

"What if we changed fetch mode to fetch all rows at once with fetchAll()? How would the time complexity change?"

Practice

1. Which PHP PDO fetch mode returns each row as an associative array with column names as keys?

easy

A. PDO::FETCH_OBJ

B. PDO::FETCH_NUM

C. PDO::FETCH_ASSOC

D. PDO::FETCH_COLUMN

Solution

1. Step 1: Understand fetch modes

   PDO::FETCH_ASSOC returns rows as associative arrays with column names as keys.

2. Step 2: Compare with other modes

   PDO::FETCH_NUM returns numeric arrays, PDO::FETCH_OBJ returns objects, PDO::FETCH_COLUMN returns a single column value.

3. Final Answer:

   PDO::FETCH_ASSOC -> Option C

4. Quick Check:

   Associative array = PDO::FETCH_ASSOC [OK]

Hint: Associative arrays use PDO::FETCH_ASSOC for column-name keys [OK]

Common Mistakes:

• Confusing PDO::FETCH_NUM with associative arrays
• Using PDO::FETCH_OBJ when array is needed
• Thinking PDO::FETCH_COLUMN returns full rows

2. Which of the following is the correct syntax to fetch a single row as an object using PDO?

easy

A. $stmt->fetch(PDO::FETCH_OBJ);

B. $stmt->fetch(PDO::FETCH_ASSOC);

C. $stmt->fetchAll(PDO::FETCH_OBJ);

D. $stmt->fetchColumn();

Solution

1. Step 1: Identify fetch method for single row as object

   fetch() returns one row; PDO::FETCH_OBJ returns it as an object.

2. Step 2: Check other options

   fetchAll() returns all rows, fetchColumn() returns a single column value, fetch(PDO::FETCH_ASSOC) returns an associative array.

3. Final Answer:

   $stmt->fetch(PDO::FETCH_OBJ); -> Option A

4. Quick Check:

   Single row object = fetch(PDO::FETCH_OBJ) [OK]

Hint: Use fetch(PDO::FETCH_OBJ) for one row as object [OK]

Common Mistakes:

• Using fetchAll() when only one row is needed
• Using fetchColumn() to get full row
• Confusing FETCH_ASSOC with FETCH_OBJ

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

$stmt = $pdo->query('SELECT id, name FROM users LIMIT 1');
$row = $stmt->fetch(PDO::FETCH_NUM);
echo $row[0] . ', ' . $row[1];

medium

A. id, name

B. 1, Alice

C. Array, Array

D. Error: Undefined index

Solution

1. Step 1: Understand PDO::FETCH_NUM

   fetch(PDO::FETCH_NUM) returns a numeric array with column values indexed 0,1,...

2. Step 2: Analyze echo statement

   $row[0] is the first column (id), $row[1] is the second column (name). Assuming first user has id=1 and name='Alice', output is '1, Alice'.

3. Final Answer:

   1, Alice -> Option B

4. Quick Check:

   Numeric array indexes = 0,1 values [OK]

Hint: FETCH_NUM returns numeric keys, use indexes 0,1 for columns [OK]

Common Mistakes:

• Expecting column names as keys with FETCH_NUM
• Using associative keys with numeric array
• Assuming fetchColumn returns full row

4. Identify the error in this PHP code snippet that fetches all user names as a single column array:

$stmt = $pdo->query('SELECT name FROM users');
$names = $stmt->fetch(PDO::FETCH_COLUMN);
print_r($names);

medium

A. print_r cannot print arrays

B. PDO::FETCH_COLUMN is invalid fetch mode

C. Query should select id, not name

D. fetch() returns only one value, should use fetchAll()

Solution

1. Step 1: Understand fetch() vs fetchAll()

   fetch() returns a single row or value, fetchAll() returns all rows.

2. Step 2: Check usage of FETCH_COLUMN

   FETCH_COLUMN fetches a single column value per row; to get all names, fetchAll(PDO::FETCH_COLUMN) is needed.

3. Final Answer:

   fetch() returns only one value, should use fetchAll() -> Option D

4. Quick Check:

   fetchAll(PDO::FETCH_COLUMN) gets all column values [OK]

Hint: Use fetchAll(PDO::FETCH_COLUMN) to get all values of one column [OK]

Common Mistakes:

• Using fetch() instead of fetchAll() for multiple rows
• Thinking FETCH_COLUMN fetches all rows automatically
• Misunderstanding print_r usage

5. You want to fetch all rows from a database as objects but only include rows where the 'active' column is true. Which PHP code snippet correctly applies this filter using fetch modes and styles?

hard

A. $rows = $stmt->fetchAll(PDO::FETCH_OBJ); $activeRows = array_filter($rows, fn($r) => $r->active);

B. $rows = $stmt->fetchAll(PDO::FETCH_ASSOC); foreach ($rows as $r) { if ($r['active']) echo $r; }

C. $rows = $stmt->fetchAll(PDO::FETCH_COLUMN); $activeRows = array_filter($rows, fn($r) => $r->active);

D. $rows = $stmt->fetch(PDO::FETCH_OBJ); while ($rows->active) { echo $rows; }

Solution

1. Step 1: Fetch all rows as objects

   fetchAll(PDO::FETCH_OBJ) returns an array of objects, each representing a row.

2. Step 2: Filter rows where 'active' is true

   array_filter with a callback checks the 'active' property on each object, keeping only those with true.

3. Step 3: Check other options

   $rows = $stmt->fetchAll(PDO::FETCH_ASSOC); foreach ($rows as $r) { if ($r['active']) echo $r; } uses associative arrays but does not collect filtered rows properly; $rows = $stmt->fetchAll(PDO::FETCH_COLUMN); $activeRows = array_filter($rows, fn($r) => $r->active); uses FETCH_COLUMN which returns single column values, not objects; $rows = $stmt->fetch(PDO::FETCH_OBJ); while ($rows->active) { echo $rows; } misuses fetch() and while loop.

4. Final Answer:

   $rows = $stmt->fetchAll(PDO::FETCH_OBJ); $activeRows = array_filter($rows, fn($r) => $r->active); -> Option A

5. Quick Check:

   fetchAll(PDO::FETCH_OBJ) + array_filter for property check [OK]

Hint: Use fetchAll(PDO::FETCH_OBJ) then array_filter by property [OK]

Common Mistakes:

• Using fetch() instead of fetchAll() for multiple rows
• Using FETCH_COLUMN when objects are needed
• Trying to filter before fetching all rows