SQL type casting rules

1. details docs about type casting, have a look at https://docs.databricks.com/en/sql/language-manual/functions/cast.html
2. define casting rules: **safe casting** and **unsafe casting**
- Exception handler for unsafe casting ?, e.g precision lost

Review the existing SQL documentation in the project, likely under docs/ or a similar directory. Examine the codebase for SQL type casting implementation to understand current behavior. Define safe and unsafe casting rules based on the Databricks reference, and propose where to add this documentation. Check if there are existing tests for casting to see examples.

索引モデルが issue の本文から書いたものです。