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Add documentation to clarify #530 Wrong number of statement arguments with "like '%?%'" #531

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Add documentation to clarify #530 Wrong number of statement arguments with "like '%?%'" #531

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21 changes: 21 additions & 0 deletions README.md
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Expand Up @@ -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)
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```


## ? as a parameter

SQLite only understands `?` as a parameter when it is a placeholder for a whole value (int, double, string, blob, null).
On the other side, the `?` in `'%?%'` is just a character in the `'%?%'` string value: it is not a query parameter, and is not processed in any way.
You need to take this into consideration for queries like:

```sql
SELECT * FROM students where name like '%Robert%';
```

To provide the parameter, you can 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])
}
```


## Error Handling

GRDB can throw [DatabaseError](#databaseerror), [PersistenceError](#persistenceerror), or crash your program with a [fatal error](#fatal-errors).
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