@@ -85,56 +85,45 @@ SQLClean in other configurations, on the same files:
 ## Custom Rules in Python
 
 A custom Rule is a Python function over each statement, already parsed and located by the Rust
-engine. This one enforces a team convention, not a rule of SQL: the columns the team lists in
-`EMAIL_COLUMNS` are compared case-insensitively. An `=` or `<>` that reads one of them, bare or as
-`LOWER(email)`, passes only when both sides are `LOWER(...)` or a lower-case string literal. It
-reads comparisons from the statement's syntax tree, so comments and string literals never match,
-and it only reports, because changing a comparison is the author's call.
+engine. This team uses `COALESCE` instead of `IFNULL` to keep queries portable. The Rule reads
+function facts, not SQL text, so comments and string literals don't match. It only reports;
+the author chooses the replacement.
 
 ```python
 # sql_rules/team.py
-import polyglot_sql as ast  # pip install "sqlclean[ast]"
-
 from sqlclean import Finding, RuleContext, Statement, rule
 
-EMAIL_COLUMNS = {"email", "billing_email"}  # the columns this team treats as email addresses
-
-
-def email_column(operand: ast.Expression) -> ast.Column | None:
-    """The designated email column an operand reads, written bare or as LOWER(column)."""
-    column = operand.this if isinstance(operand, ast.Lower) else operand
-    is_email = isinstance(column, ast.Column) and column.name.lower() in EMAIL_COLUMNS
-    return column if is_email else None
-
-
-def lowered(operand: ast.Expression) -> bool:
-    """Whether an operand is LOWER(...) or a string literal with no upper-case letters."""
-    literal = isinstance(operand, ast.Literal) and operand.is_string
-    return isinstance(operand, ast.Lower) or (literal and operand.name == operand.name.lower())
-
 
 @rule(
     code="XSQLCLEANTEAM001",
-    message="Email comparisons must be case-insensitive",
-    remediation="Compare LOWER(email) with LOWER(...) instead.",
+    message="Use COALESCE instead of IFNULL",
+    remediation="Write COALESCE(value, fallback).",
     enabled_by_default=True,
 )
-def case_insensitive_email(*, statement: Statement, ctx: RuleContext) -> list[Finding]:
-    starts = {
-        column.to_dict()["column"]["span"]["start"]
-        for comparison in statement.polyglot_ast().find_all(ast.Eq, ast.Neq)
-        for column in map(email_column, (comparison.this, comparison.expression))
-        if column is not None and not all(map(lowered, (comparison.this, comparison.expression)))
-    }
+def prefer_coalesce(*, statement: Statement, ctx: RuleContext) -> list[Finding]:
     return [
-        ctx.finding(subject=statement, node=column)
-        for column in statement.columns
-        if column.start - statement.start in starts
+        ctx.finding(subject=statement, node=function)
+        for function in statement.functions
+        if function.name.upper() == "IFNULL"
     ]
 ```
 
-Point `[rules] custom = ["sql_rules"]` in `sqlclean.toml` at it. The syntax tree needs the `ast`
-extra: `pip install "sqlclean[ast]"`. See
+Point `[rules] custom = ["sql_rules"]` in `sqlclean.toml` at it, then run `sqlclean check`:
+
+```sql
+SELECT IFNULL(discount, 0) FROM orders;
+```
+
+```text
+error[XSQLCLEANTEAM001]: Use COALESCE instead of IFNULL
+  --> orders.sql:1:8
+   |
+ 1 | SELECT IFNULL(discount, 0) FROM orders;
+   |        ^^^^^^
+  = help: Write COALESCE(value, fallback).
+```
+
+No extra dependencies are needed. See
 [custom Rules](https://sqlclean.io/docs/custom-rules/overview/),
 [statement facts](https://sqlclean.io/docs/custom-rules/statement-facts/) and
 [testing custom Rules](https://sqlclean.io/docs/custom-rules/testing/).
 
@@ -21,21 +21,25 @@ A custom Rule is a Python function decorated with `@rule`. It receives a stateme
 whole repository, and returns findings. Custom Rules use the same selection, exceptions, fixes,
 output and cache as the built-in Rules.
 
+This team uses `COALESCE` instead of `IFNULL` to keep queries portable. The Rule reads function
+facts, so comments and string literals don't match. It only reports, leaving the replacement to
+the author. No extra dependencies are needed.
+
 ```python
-# sql_rules/naming.py
+# sql_rules/team.py
 from sqlclean import Finding, RuleContext, Statement, rule
 
 @rule(
-    code="XSQLCLEANNAME001",
-    message="Query reads a temporary table",
-    remediation="Read from the published table instead.",
+    code="XSQLCLEANTEAM001",
+    message="Use COALESCE instead of IFNULL",
+    remediation="Write COALESCE(value, fallback).",
     enabled_by_default=True,
 )
-def temporary_table(*, statement: Statement, ctx: RuleContext) -> list[Finding]:
+def prefer_coalesce(*, statement: Statement, ctx: RuleContext) -> list[Finding]:
     return [
-        ctx.finding(subject=statement, node=table)
-        for table in statement.tables
-        if table.name.startswith("tmp_")
+        ctx.finding(subject=statement, node=function)
+        for function in statement.functions
+        if function.name.upper() == "IFNULL"
     ]
 ```
 
@@ -46,15 +50,23 @@ Point `[rules] custom` at the file or its directory:
 custom = ["sql_rules"]
 ```
 
+Run `sqlclean check` on `orders.sql`:
+
+```sql
+SELECT IFNULL(discount, 0) FROM orders;
+```
+
 ```text
-error[XSQLCLEANNAME001]: Query reads a temporary table
-  --> models/orders.sql:2:6
+error[XSQLCLEANTEAM001]: Use COALESCE instead of IFNULL
+  --> orders.sql:1:8
    |
- 2 | FROM tmp_orders
-   |      ^^^^^^^^^^
-  = help: Read from the published table instead.
+ 1 | SELECT IFNULL(discount, 0) FROM orders;
+   |        ^^^^^^
+  = help: Write COALESCE(value, fallback).
 ```
 
+Write `SELECT COALESCE(discount, 0) FROM orders;` to satisfy this convention.
+
 ## Where Rules live
 
 `[rules] custom` lists Python files or directories, relative to the configuration root. SQLClean
@@ -63,8 +75,8 @@ functions, constants and classes stay ordinary Python.
 
 - Files named `test_*.py`, `conftest.py` and `__init__.py` are not loaded as Rules, so tests can
   live next to the Rules they test.
-- Modules are imported by their path from the configuration root: `sql_rules/naming.py` is
-  `sql_rules.naming`. A Rule module can import a helper the same way, for example
+- Modules are imported by their path from the configuration root: `sql_rules/team.py` is
+  `sql_rules.team`. A Rule module can import a helper the same way, for example
   `from sql_rules.helpers import table_prefixes`.
 - Two Rules with the same code are an error: `custom Rule code XSQLCLEANNAME001 is defined twice`.
 
 
@@ -42,7 +42,7 @@ packages = ["src/sqlclean"]
 dev = [
   "commitizen>=4.9.1",
   "fensu>=0.28.0",
-  "polyglot-sql-chio==0.13.10", # the ast extra, for the README custom Rule test
+  "polyglot-sql-chio==0.13.10", # optional AST API
   "pre-commit>=4.3.0",
   "pytest>=8.4.2",
   "pytest-xdist>=3.8.0",
 
@@ -30,7 +30,7 @@ class HarnessFailureTestCase:
 
 @dataclass(frozen=True)
 class ReadmeRuleTestCase:
-    """SQL for the README custom Rule and the text of every column it must report."""
+    """SQL for the introductory custom Rule and the text of every function it must report."""
 
     description: str
     source: str
 
@@ -3,10 +3,14 @@
 from __future__ import annotations
 
 import importlib.util
+import re
+from html import unescape
 from importlib.machinery import ModuleSpec
 from pathlib import Path
 from types import ModuleType
 
+import pytest
+
 from sqlclean.rules.types import RuleCheck
 
 SAMPLE_RULES: str = """
@@ -106,19 +110,32 @@ def sample_rule(directory: Path, name: str) -> RuleCheck:
 
 
 README: Path = Path(__file__).resolve().parents[5] / "README.md"
-BLOCK_START: str = "```python\n# sql_rules/team.py\n"
+INTRODUCTIONS: tuple[Path, ...] = (
+    README,
+    README.parent / "website/public/index.html",
+    README.parent / "website/src/content/docs/docs/custom-rules/overview.mdx",
+)
+BLOCK_START: str = "# sql_rules/team.py\n"
+
+
+@pytest.fixture(params=INTRODUCTIONS, ids=lambda path: path.name)
+def document(request: pytest.FixtureRequest) -> Path:
+    """Run each case against every published introductory snippet."""
+
+    path: Path = request.param
+    return path
 
 
-def readme_rule(directory: Path) -> RuleCheck:
-    """Return the README's case_insensitive_email Rule, imported from its code block."""
+def readme_rule(directory: Path, document: Path = README) -> RuleCheck:
+    """Import prefer_coalesce from an introductory example, removing HTML markup if needed."""
 
-    text: str = README.read_text(encoding="utf-8")
+    text: str = unescape(re.sub(r"</?span\b[^>]*>", "", document.read_text(encoding="utf-8")))
     start: int = text.index(BLOCK_START) + len(BLOCK_START)
     path: Path = directory / "readme_team.py"
-    path.write_text(text[start : text.index("```", start)], encoding="utf-8")
+    path.write_text(text[start:].split("```")[0].split("# sqlclean.toml:")[0], encoding="utf-8")
     spec: ModuleSpec | None = importlib.util.spec_from_file_location("readme_team", path)
     assert spec is not None and spec.loader is not None
     module: ModuleType = importlib.util.module_from_spec(spec)
     spec.loader.exec_module(module)
-    check: RuleCheck = module.case_insensitive_email
+    check: RuleCheck = module.prefer_coalesce
     return check
@@ -1,4 +1,4 @@
-"""The custom Rule example in README.md, run as written through the Rule harness."""
+"""The introductory custom Rule examples, run as written through the Rule harness."""
 
 from __future__ import annotations
 
@@ -9,66 +9,48 @@
 from sqlclean.rules.types import RuleCheck
 from sqlclean.testing import RuleCase, RuleResult, evaluate_rule
 from tests.integration.src.sqlclean.rule_testing._test_types import ReadmeRuleTestCase
+from tests.integration.src.sqlclean.rule_testing.helpers import document as document
 from tests.integration.src.sqlclean.rule_testing.helpers import readme_rule
 
 
 @pytest.mark.parametrize(
     "test_case",
     [
         ReadmeRuleTestCase(
-            description="a bare email column compared with a mixed-case literal",
-            source="SELECT id FROM customers c WHERE c.email = 'Alice@example.com'",
-            expected_texts=("c.email",),
+            description="the documented SQL reports the function name",
+            source="SELECT IFNULL(discount, 0) FROM orders;",
+            expected_texts=("IFNULL",),
         ),
         ReadmeRuleTestCase(
-            description="LOWER on one side only still compares with a mixed-case literal",
-            source="SELECT id FROM customers WHERE LOWER(email) = 'Alice@Example.com'",
-            expected_texts=("email",),
+            description="mixed-case calls nested in a CTE and subquery are reported",
+            source="WITH totals AS (SELECT ifnull(discount, 0) AS discount FROM orders) "
+            "SELECT (SELECT IfNull(MAX(discount), 0) FROM totals)",
+            expected_texts=("ifnull", "IfNull"),
         ),
         ReadmeRuleTestCase(
-            description="a designated column on the right of <> in upper case",
-            source="SELECT id FROM customers WHERE 'Carol@example.com' <> BILLING_EMAIL",
-            expected_texts=("BILLING_EMAIL",),
+            description="a later statement after a multibyte literal uses file character offsets",
+            source="SELECT 'é';\nSELECT IFNULL(discount, 0) FROM orders;",
+            expected_texts=("IFNULL",),
         ),
         ReadmeRuleTestCase(
-            description="UPPER does not normalize the other side to lower case",
-            source="SELECT id FROM customers WHERE LOWER(email) = UPPER('x')",
-            expected_texts=("email",),
-        ),
-        ReadmeRuleTestCase(
-            description="a bare column against a lower-case literal is still not normalized",
-            source="SELECT id FROM customers WHERE email = 'carol@example.com'",
-            expected_texts=("email",),
-        ),
-        ReadmeRuleTestCase(
-            description="a comparison inside a CTE after a multibyte literal",
-            source="WITH u AS (SELECT 'é' AS m, email FROM users WHERE email = 'X@y.com') "
-            "SELECT * FROM u",
-            expected_texts=("email",),
-        ),
-        ReadmeRuleTestCase(
-            description="LOWER on both sides passes",
-            source="SELECT * FROM u JOIN a ON LOWER(u.email) = LOWER(a.email)",
-            expected_texts=(),
-        ),
-        ReadmeRuleTestCase(
-            description="LOWER against a lower-case literal passes",
-            source="SELECT id FROM customers WHERE LOWER(email) = 'bob@example.com'",
+            description="the recommended COALESCE replacement passes",
+            source="SELECT COALESCE(discount, 0) FROM orders;",
             expected_texts=(),
         ),
         ReadmeRuleTestCase(
-            description="comments, strings, other columns, LIKE and projections never match",
-            source="-- email = 'A@b.c'\nSELECT email, 'email = x' AS note FROM customers "
-            "WHERE email_verified = TRUE AND email LIKE '%dave%'",
+            description="comments, strings, identifiers and longer function names never match",
+            source="-- IFNULL(discount, 0)\n"
+            "SELECT 'IFNULL(discount, 0)' AS note, \"IFNULL\", "
+            "my_ifnull(discount, 0) FROM orders /* IFNULL(discount, 0) */",
             expected_texts=(),
         ),
     ],
     ids=lambda case: case.description,
 )
-def test_given_readme_email_rule_when_evaluating_then_unnormalized_comparisons_are_reported(
-    tmp_path: Path, test_case: ReadmeRuleTestCase
+def test_given_introductory_rule_when_evaluating_then_only_ifnull_calls_are_reported(
+    tmp_path: Path, document: Path, test_case: ReadmeRuleTestCase
 ) -> None:
-    rule: RuleCheck = readme_rule(tmp_path)
+    rule: RuleCheck = readme_rule(tmp_path, document)
 
     result: RuleResult = evaluate_rule(
         rule=rule,
@@ -84,4 +66,5 @@ def test_given_readme_email_rule_when_evaluating_then_unnormalized_comparisons_a
     )
 
     assert texts == test_case.expected_texts
+    assert all(finding.edits == () for finding in result.findings)
     assert result.fixed_source == test_case.source