Fetching results (fetch, fetchAll) in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the fetch() method do in PHP PDO?

The fetch() method retrieves the next row from a result set as an array or object. It returns one row at a time or false if no more rows are available.

beginner

How is fetchAll() different from fetch()?

fetchAll() retrieves all remaining rows from the result set at once as an array of arrays or objects, while fetch() gets one row at a time.

intermediate

What is a common use case for fetch()?

Use fetch() when you want to process rows one by one, for example, in a loop to handle large data sets without loading everything into memory.

beginner

What does fetchAll(PDO::FETCH_ASSOC) return?

It returns all rows as an array of associative arrays, where each array uses column names as keys.

beginner

What happens if you call fetch() after all rows have been fetched?

It returns false, indicating there are no more rows to fetch.

Which method fetches all rows from a PDO statement at once?

A prepare()

B fetch()

C execute()

D fetchAll()

fetchAll() retrieves all rows at once, while fetch() retrieves one row at a time.

What does fetch() return when no more rows are available?

A false

B null

C empty array

D 0

fetch() returns false when there are no more rows to fetch.

Which fetch style returns rows as associative arrays with column names as keys?

A PDO::FETCH_NUM

B PDO::FETCH_OBJ

C PDO::FETCH_ASSOC

D PDO::FETCH_BOTH

PDO::FETCH_ASSOC returns rows as associative arrays keyed by column names.

When is it better to use fetch() instead of fetchAll()?

A When you want all rows at once

B When processing large data sets row by row

C When you want to prepare a statement

D When you want to execute a query

fetch() is better for large data sets to avoid loading all rows into memory at once.

What type of value does fetchAll() return?

A An array of rows

B An integer

C A boolean

D A single row

fetchAll() returns an array containing all rows from the result set.

Explain the difference between fetch() and fetchAll() in PHP PDO.

Think about how many rows each method returns and memory usage.

Describe a scenario where using fetch() is better than fetchAll().

Consider memory and performance when handling many rows.

Practice

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