Documentation: add "Extensions to SQL" page

examples/official-site/extensions-to-sql.md

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+# Extensions to SQL
+
+SQLPage makes some special treatment before executing your SQL queries.
+
+When executing your SQL file, SQLPage executes each query one at a time.
+It doesn't send the whole file as-is to the database engine.
+
+## Performance
+
+See the [performance page](/performance.sql) for details on the optimizations
+made to run your queries as fast as possible.
+
+## Variables
+
+SQL doesn't have its own mechanism for variables.
+SQLPage implements variables in the following way:
+
+### POST parameters
+
+When sending a POST request, most often by sending a form with the
+[form component](/component.sql?component=form), the form data is made
+available as variables prefixed by a semi-colon.
+
+So when this form is sent:
+
+`form.sql`
+```sql
+SELECT
+    'form' AS component,
+    'POST' AS method, -- form defaults to using the HTTP POST method
+    'result.sql' AS action;
+
+SELECT
+    'age' AS name,
+    'How old are you?' AS label,
+    'number' AS type;
+```
+
+It will make a request to this page:
+
+`result.sql`
+```sql
+SELECT
+    'text' AS component,
+    'You are ' || :age || ' years old!' AS contents;
+```
+
+`:age` will be substituted by the actual value of the POST parameter.
+
+### URL parameters
+
+Likewise, URL parameters are available as variables prefixed by a dollar sign.
+
+> URL parameters are often called GET parameters because they can originate
+> from a form with 'GET' as the method.
+
+So the previous example can be reworked to handle URL parameters:
+
+`result.sql`
+```sql
+SELECT
+    'text' AS component,
+    'You are ' || $age || ' years old!' AS contents;
+```
+
+By querying this page with this URL: `/request.sql?age=42`
+we would get `You are 42 years old!` as a response.
+
+### The `SET` command
+
+SQLPage overrides the behavior of `SET` statements in SQL to store variables in SQLPage itself instead of running the statement on the database. 
+
+```sql
+SET coalesced_post_id = COALESCE($post_id, 0);
+```
+
+`SET` statements are transformed into `SELECT` queries, and their result is stored in a `$`-variable:
+
+```sql
+SELECT COALESCE($post_id, 0);
+```
+
+We can override a previous `$`-variable:
+
+```sql
+SET post_id = COALESCE($post_id, 0);
+```
+
+### Limitations
+
+`$`-variables and `:`-variables are stored by SQLPage, not in the database.
+
+They can only store a string, or null.
+
+As such, they're not designed to store table-valued results.
+They will only store the first value of the first column:
+
+```sql
+CREATE TABLE t(a, b);
+INSERT INTO t(a, b) VALUES (1, 2), (3, 4);
+
+SET var = (SELECT * FROM t);
+
+-- now $var contains '1'
+```
+
+Temporary table-valued results can be stored in two ways:
+
+1. Using temporary tables
+
+This is the most efficient method to store large values.
+```sql
+-- Database connections are reused and temporary tables are stored at the
+-- connection level, so we make sure the table doesn't exist already
+DROP TABLE IF EXISTS my_temp_table;
+CREATE TEMPORARY TABLE my_temp_table AS
+SELECT a, b
+FROM my_stored_table ...
+
+-- Insert data from direct values
+INSERT INTO my_temp_table(a, b)
+VALUES (1, 2), (3, 4);
+```
+
+2. Using JSON
+
+This can be more convenient, but should only be used for small values, because data
+is copied from the database into SQLPage memory, and to the database again at each use.
+
+You can use the [JSON functions from your database](/blog.sql?post=JSON in SQL%3A A Comprehensive Guide).
+Here are some examples with SQLite:
+```sql
+-- CREATE TABLE my_table(a, b);
+-- INSERT INTO my_table(a, b)
+-- VALUES (1, 2), (3, 4);
+
+SET my_json = (
+    SELECT json_group_array(a)
+    FROM my_table
+);
+-- [1, 3]
+
+SET my_json = json_array(1, 2, 3);
+-- [1, 2, 3]
+```
+
+## Functions
+
+Functions starting with `sqlpage.` are executed by SQLPage, not by your database engine.
+See the [functions page](/functions.sql) for more details.
+
+They're either executed before or after the query is run in the database.
+
+### Execution functions *before* sending a query to the database
+
+When they don't process results coming from the database:
+
+```sql
+SELECT * FROM blog WHERE slug = sqlpage.path()
+```
+
+`sqlpage.path()` will get replaced by the result of the function.
+
+### Execution functions *after* receiving results from the database
+
+When they process results coming from the database:
+
+```sql
+SELECT sqlpage.read_file_as_text(blog_post_file) AS title FROM blog;
+```
+
+The query executed will be:
+
+```sql
+SELECT blog_post_file AS title FROM blog;
+```
+
+Then `sqlpage.read_file_as_text()` will be called on each row.
+
+## Implementation details of variables and functions
+
+All queries run by SQLPage in the database are first prepared, then executed.
+
+Statements are prepared and cached the first time they're encountered by SQLPage.
+Then those cached prepared statements are executed at each run, with parameter substitution.
+
+All variables and function results are cast as text, to let the
+database query optimizer know only strings (or nulls) will be passed.
+
+Examples:
+
+```sql
+-- Source query
+SELECT * FROM blog WHERE slug = sqlpage.path();
+
+-- Prepared statement (SQLite syntax)
+SELECT * FROM blog WHERE slug = CAST(?1 AS TEXT)
+```
+
+```sql
+-- Source query
+SET post_id = COALESCE($post_id, 0);
+
+-- Prepared statement (SQLite syntax)
+SELECT COALESCE(CAST(?1 AS TEXT), 0)
+```

examples/official-site/extensions-to-sql.sql

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+select 'http_header' as component,
+	'public, max-age=300, stale-while-revalidate=3600, stale-if-error=86400' as "Cache-Control";
+
+select 'dynamic' as component, properties FROM example WHERE component = 'shell' LIMIT 1;
+
+-- Article by Matthew Larkin
+select 'text' as component,
+    sqlpage.read_file_as_text('extensions-to-sql.md') as contents_md,
+    false as article;

examples/official-site/sqlpage/migrations/01_documentation.sql

@@ -1231,6 +1231,7 @@ You see the [page layouts demo](./examples/layouts.sql) for a live example of th
                     {"link": "/your-first-sql-website", "title": "Getting started", "icon": "book"},
                     {"link": "/components.sql", "title": "All Components", "icon": "list-details"},
                     {"link": "/functions.sql", "title": "SQLPage Functions", "icon": "math-function"},
+                    {"link": "/extensions-to-sql.sql", "title": "Extensions to SQL", "icon": "cube-plus"},
                     {"link": "/custom_components.sql", "title": "Custom Components", "icon": "puzzle"},
                     {"link": "//github.com/sqlpage/SQLPage/blob/main/configuration.md#configuring-sqlpage", "title": "Configuration", "icon": "settings"}
                 ]}