Add FAQ to clarify "Wrong number of statement arguments" error with "like '%?%'"

README.md

@@ -289,6 +289,7 @@ Documentation
 #### Good to Know
 
 - [Avoiding SQL Injection](#avoiding-sql-injection)
+- [? as a parameter](#-as-a-parameter)
 - [Error Handling](#error-handling)
 - [Unicode](#unicode)
 - [Memory Management](#memory-management)
@@ -8506,6 +8507,7 @@ FAQ
 - [Generic parameter 'T' could not be inferred](#generic-parameter-t-could-not-be-inferred)
 - [SQLite error 10 "disk I/O error", SQLite error 23 "not authorized"](#sqlite-error-10-disk-io-error-sqlite-error-23-not-authorized)
 - [What Are Experimental Features?](#what-are-experimental-features)
+- [SQLite error 21 "wrong number of statement arguments" with LIKE queries](#sqlite-error-21-wrong-number-of-statement-arguments-with-like-queries)
 
 
 ### How do I create a database in my application?
@@ -8645,6 +8647,29 @@ There is an exception, though: *experimental features*, marked with the "**:fire
 Those experimental features are not protected by semantic versioning, and may break between two minor releases of the library. To help them becoming stable, [your feedback](https://github.com/groue/GRDB.swift/issues) is greatly appreciated.
 
 
+### SQLite error 21 "wrong number of statement arguments" with LIKE queries
+
+You may get the error "wrong number of statement arguments" when executing a LIKE query similar to:
+
+```swift
+let name = textField.text
+try dbQueue.read { db in
+    try db.execute(sql: "SELECT * FROM students where title like '%?%'", arguments: [name])
+}
+```
+
+What is important to understand here is that SQLite only interprets `?` as a parameter when it is a placeholder for a whole value (int, double, string, blob, null). In this incorrect query, `?` is just a character in the `'%?%'` string value: it is not a query parameter, and is not processed in any way. See [https://www.sqlite.org/lang_expr.html#varparam](https://www.sqlite.org/lang_expr.html#varparam) for more information about SQLite parameters.
+
+To provide the parameter, one option is to let SQLite build the like pattern using the string concatenation operator `||`:
+
+```swift
+let name = textField.text
+try dbQueue.read { db in
+    try db.execute(sql: "SELECT * FROM students where name like '%' || ? || '%'", arguments: [name])
+}
+```
+
+
 Sample Code
 ===========