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Fetching results (fetch, fetchAll) in PHP - Time & Space Complexity

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Time Complexity: Fetching results (fetch, fetchAll)
O(n)
Understanding Time Complexity

When we get data from a database in PHP, we often use fetch or fetchAll to retrieve rows. Understanding how long these operations take helps us write faster programs.

We want to know: how does the time to get results grow as the number of rows grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.


// Assume $stmt is a prepared PDO statement
$stmt->execute();

// Fetch one row at a time
while ($row = $stmt->fetch()) {
    // process $row
}

// Or fetch all rows at once
$allRows = $stmt->fetchAll();

This code fetches rows from a database query, either one by one or all at once.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

  • Primary operation: Fetching each row from the database result set.
  • How many times: Once for each row in the result (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
1010 fetch operations
100100 fetch operations
10001000 fetch operations

Pattern observation: The time grows directly with the number of rows; double the rows, double the work.

Final Time Complexity

Time Complexity: O(n)

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

Common Mistake

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

[OK] Correct: Fetching all rows uses more memory and can be slower if the result set is very large. Both methods still take time proportional to the number of rows.

Interview Connect

Understanding how fetching data scales helps you write efficient database code, a skill useful in many real projects and interviews.

Self-Check

"What if we used fetchAll but limited the query to only 10 rows? How would the time complexity change?"

Practice

(1/5)
1. What does the fetch() method do when used with a PDO statement in PHP?
easy
A. Retrieves one row from the query result
B. Retrieves all rows from the query result
C. Executes the SQL query
D. Closes the database connection

Solution

  1. Step 1: Understand the purpose of fetch()

    The fetch() method is designed to get a single row from the result set of a database query.

  2. Step 2: Compare with other methods

    Unlike fetchAll(), which retrieves all rows, fetch() only returns one row at a time.

  3. Final Answer:

    Retrieves one row from the query result -> Option A

  4. Quick Check:

    fetch() = one row [OK]
Hint: fetch() gets one row, fetchAll() gets all rows [OK]
Common Mistakes:
  • Confusing fetch() with fetchAll()
  • Thinking fetch() executes the query
  • Assuming fetch() closes the connection
2. Which of the following is the correct syntax to fetch all rows from a PDO statement $stmt in PHP?
easy
A. $rows = $stmt->fetchAll();
B. $rows = $stmt->fetch_all();
C. $rows = fetchAll($stmt);
D. $rows = $stmt->fetch();

Solution

  1. Step 1: Identify the correct method name

    The PDOStatement object uses fetchAll() to retrieve all rows as an array.

  2. Step 2: Check method call syntax

    The correct syntax is $stmt->fetchAll(); with camelCase and parentheses.

  3. Final Answer:

    $rows = $stmt->fetchAll(); -> Option A

  4. Quick Check:

    fetchAll() method syntax correct [OK]
Hint: Use fetchAll() with parentheses on the statement object [OK]
Common Mistakes:
  • Using fetch_all() instead of fetchAll()
  • Calling fetchAll() as a standalone function
  • Using fetch() when fetchAll() is needed
3. Given the following PHP code, what will be the output?
$stmt = $pdo->query('SELECT id FROM users LIMIT 2');
$row1 = $stmt->fetch();
$row2 = $stmt->fetch();
$row3 = $stmt->fetch();
var_dump([$row1, $row2, $row3]);
medium
A. [false, false, false]
B. [array with first row, array with second row, array with third row]
C. [array with first row, array with second row, false]
D. Syntax error

Solution

  1. Step 1: Understand fetch() behavior on limited rows

    The query returns 2 rows. Each fetch() call returns one row until no rows remain.

  2. Step 2: Analyze each fetch() call

    First fetch() returns first row array, second fetch() returns second row array, third fetch() returns false (no more rows).

  3. Final Answer:

    [array with first row, array with second row, false] -> Option C

  4. Quick Check:

    fetch() returns false after last row [OK]
Hint: fetch() returns false when no rows left [OK]
Common Mistakes:
  • Expecting fetch() to return null instead of false
  • Assuming fetch() returns empty array after last row
  • Thinking fetch() fetches all rows at once
4. What is wrong with this PHP code snippet?
$stmt = $pdo->query('SELECT name FROM users');
$rows = $stmt->fetch();
foreach ($rows as $row) {
    echo $row['name'];
}
medium
A. fetch() returns all rows, so foreach is unnecessary
B. The variable $rows is not defined
C. The query syntax is invalid
D. fetch() returns a single row, so foreach on it causes error

Solution

  1. Step 1: Check what fetch() returns

    fetch() returns a single row as an array, not an array of rows.

  2. Step 2: Understand foreach usage

    foreach expects an array of rows, but here it loops over columns of one row, causing errors when accessing $row['name'].

  3. Final Answer:

    fetch() returns a single row, so foreach on it causes error -> Option D

  4. Quick Check:

    fetch() = one row, foreach needs multiple rows [OK]
Hint: Use fetchAll() when looping over multiple rows [OK]
Common Mistakes:
  • Using fetch() when fetchAll() is needed for loops
  • Assuming fetch() returns all rows
  • Not checking data structure before foreach
5. You want to fetch all users from the database but only keep those whose age is greater than 18. Which code snippet correctly uses fetchAll() with a filter in PHP?
hard
A. $adults = $stmt->fetchAll(fn($r) => $r['age'] > 18);
B. $rows = $stmt->fetchAll(); $adults = array_filter($rows, fn($r) => $r['age'] > 18);
C. $adults = $stmt->fetchAll(); foreach ($adults as $r) { if ($r['age'] <= 18) unset($r); }
D. $adults = $stmt->fetch(); while ($r['age'] > 18) { $adults[] = $r; }

Solution

  1. Step 1: Fetch all rows first

    Use fetchAll() to get all rows as an array.

  2. Step 2: Filter rows by age

    Use array_filter() with a callback to keep only rows where age > 18.

  3. Final Answer:

    $rows = $stmt->fetchAll(); $adults = array_filter($rows, fn($r) => $r['age'] > 18); -> Option B

  4. Quick Check:

    fetchAll() + array_filter() = filtered rows [OK]
Hint: fetchAll() then filter with array_filter() for conditions [OK]
Common Mistakes:
  • Passing a callback directly to fetchAll() (not supported)
  • Modifying array inside foreach without reassigning
  • Using fetch() when multiple rows needed
Recommended

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