Fetch modes and styles in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the default fetch mode in PHP's PDO when retrieving data?

The default fetch mode is PDO::FETCH_BOTH, which returns an array indexed by both column name and number.

beginner

Explain PDO::FETCH_ASSOC fetch mode.

PDO::FETCH_ASSOC returns an array indexed by column names only, which helps avoid duplicate data and is memory efficient.

intermediate

What does PDO::FETCH_OBJ do when fetching data?

It returns each row as an anonymous object where columns are accessible as properties, e.g., $row->columnName.

intermediate

How does PDO::FETCH_CLASS differ from other fetch modes?

It fetches data into an instance of a specified class, mapping columns to class properties, allowing object-oriented data handling.

intermediate

What is the purpose of PDO::FETCH_COLUMN?

It fetches only a single column from the next row in the result set, useful when you need just one column's data.

Which fetch mode returns data as an object with properties matching column names?

A PDO::FETCH_COLUMN

B PDO::FETCH_ASSOC

C PDO::FETCH_NUM

D PDO::FETCH_OBJ

PDO::FETCH_OBJ returns each row as an object with properties named after the columns.

If you want to fetch data as an array indexed by column names only, which fetch mode should you use?

A PDO::FETCH_BOTH

B PDO::FETCH_NUM

C PDO::FETCH_ASSOC

D PDO::FETCH_CLASS

PDO::FETCH_ASSOC returns an associative array indexed by column names.

What does PDO::FETCH_NUM return?

A An array indexed by column numbers

B A class instance

C An object with column properties

D An array indexed by column names

PDO::FETCH_NUM returns an array indexed by column numbers (0-based).

Which fetch mode allows you to fetch data directly into a user-defined class?

A PDO::FETCH_OBJ

B PDO::FETCH_CLASS

C PDO::FETCH_ASSOC

D PDO::FETCH_COLUMN

PDO::FETCH_CLASS maps the row data into an instance of a specified class.

When would you use PDO::FETCH_COLUMN?

A To fetch a single column from each row

B To fetch data as an object

C To fetch all columns as an associative array

D To fetch data as a numeric array

PDO::FETCH_COLUMN fetches only one column from each row, useful for lists of values.

Describe the main differences between PDO::FETCH_ASSOC, PDO::FETCH_NUM, and PDO::FETCH_BOTH.

Think about how the data is indexed in the returned array.

Explain how PDO::FETCH_CLASS can be used to work with database results in an object-oriented way.

Consider how this fetch mode helps organize data as objects.

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