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The debug component displays any set of values as formatted JSON. It’s invaluable for understanding what data is being passed to components during development.

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Basic Usage

Replace any component name with debug to see all its parameters: 
SELECT 'debug' AS component,
    TRUE AS my_top_level_property;

SELECT 'y' AS x, 42 AS z;
SELECT 'b' AS a, NULL AS c;
This displays: 
{
  "component": "debug",
  "my_top_level_property": true
}
{
  "x": "y",
  "z": 42
}
{
  "a": "b",
  "c": null
}

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Debugging SQLPage Functions

Inspect the output of SQLPage functions: 
SELECT 'debug' AS component, sqlpage.environment_variable('HOME');

SELECT 'debug' AS component, sqlpage.request_info();

SELECT 'debug' AS component, 
    sqlpage.cookie('session_token') AS session,
    sqlpage.path() AS current_path,
    sqlpage.request_method() AS method;

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Debugging Query Results

See exactly what your queries return: 
SELECT 'debug' AS component;
SELECT * FROM users WHERE id = :user_id;
This helps verify:
  • Column names and types
  • NULL vs empty strings
  • Numeric precision
  • JSON structure

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Debugging Form Submissions

Inspect all posted parameters: 
SELECT 'debug' AS component,
    :username AS username,
    :email AS email,
    :age AS age,
    :newsletter AS newsletter;
Or use variables: 
SELECT 'debug' AS component, sqlpage.variables('post');

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Debugging Complex JSON

When working with JSON data, debug helps verify structure: 
SELECT 'debug' AS component;
SELECT 
    json_object(
        'name', name,
        'items', (
            SELECT json_group_array(
                json_object('id', id, 'price', price)
            )
            FROM order_items
            WHERE order_items.order_id = orders.id
        )
    ) AS order_data
FROM orders
WHERE id = :order_id;

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Debugging Component Properties

Verify properties before passing to dynamic component: 
-- Instead of this:
-- SELECT 'dynamic' AS component, properties;

-- Debug first:
SELECT 'debug' AS component, properties
FROM (
    SELECT json_array(
        json_object('component', 'card'),
        json_object('title', 'Test', 'description', 'Test card')
    ) AS properties
);

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Comparing Actual vs Expected Data

Debug side-by-side comparisons: 
SELECT 'debug' AS component;

-- What we're sending
SELECT 
    'Expected' AS type,
    'John Doe' AS name,
    100 AS amount;

-- What the database has
SELECT 
    'Actual' AS type,
    name,
    amount
FROM transactions
WHERE id = :transaction_id;

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Debugging Authentication

Check authentication values: 
SELECT 'debug' AS component,
    sqlpage.basic_auth_username() AS username,
    sqlpage.basic_auth_password() AS password,
    :password AS form_password,
    (SELECT password_hash FROM users WHERE username = :username) AS stored_hash;

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Debugging URL Parameters

See all URL parameters: 
SELECT 'debug' AS component, sqlpage.variables('get');
Or specific parameters: 
SELECT 'debug' AS component,
    $id AS id_param,
    $page AS page_param,
    $filter AS filter_param;

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Production Safety

Never deploy pages with debug components to production. They expose internal data and can reveal sensitive information.

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Conditional Debug

Only show debug info to admins: 
-- Show debug only for admin users
SELECT 'debug' AS component, sqlpage.request_info()
WHERE :is_admin = '1';

-- Or based on environment
SELECT 'debug' AS component, sqlpage.request_info()
WHERE sqlpage.environment_variable('ENVIRONMENT') = 'development';

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Common Debugging Scenarios

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Empty Results

Why is my list empty? 
-- Instead of list, use debug
SELECT 'debug' AS component;
SELECT * FROM products WHERE category = :category;
Check if:
  • Query returns any rows
  • Column names match expectations
  • Data types are correct
  • NULL values are present

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Form Not Submitting Correctly

SELECT 'debug' AS component,
    :field1 AS field1,
    :field2 AS field2,
    sqlpage.request_method() AS method,
    sqlpage.post() AS all_post_data;

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Component Not Rendering

Replace the component with debug: 
-- Was: SELECT 'card' AS component;
SELECT 'debug' AS component;

SELECT 
    title,
    description,
    link
FROM items;

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Type Mismatches

SELECT 'debug' AS component;
SELECT 
    TYPEOF(id) AS id_type,
    TYPEOF(price) AS price_type,
    TYPEOF(created_at) AS date_type,
    *
FROM products
LIMIT 1;

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Tips and Tricks

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Pretty Print in Browser

The debug component automatically formats JSON with:
  • Syntax highlighting
  • Proper indentation
  • Collapsible sections (in modern browsers)

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Debug Multiple Queries

See results from multiple queries in sequence: 
SELECT 'debug' AS component;

-- Query 1
SELECT 'Orders' AS section, COUNT(*) AS count FROM orders;

-- Query 2  
SELECT 'Users' AS section, COUNT(*) AS count FROM users;

-- Query 3
SELECT 'Products' AS section, COUNT(*) AS count FROM products;

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Debug with Headers

Add context to debug output: 
SELECT 'text' AS component, 
    '## Debugging User Data' AS contents_md;

SELECT 'debug' AS component;
SELECT * FROM users WHERE id = :user_id;

SELECT 'text' AS component,
    '## Debugging Order Data' AS contents_md;

SELECT 'debug' AS component;
SELECT * FROM orders WHERE user_id = :user_id;

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No Parameters

The debug component accepts any parameters and displays them all. There are no specific parameter definitions - it simply shows whatever you pass to it.

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