Update README with supported versions, MySQL 9.x notes, and PostgreSQL details

- Add supported versions table covering MySQL, Percona, MariaDB, PostgreSQL
- Document MySQL 9.5 --commands flag and 9.6 md5/sha removal
- Explain SHA-256 as the recommended cross-database integrity test
- Expand PostgreSQL section: differences from MySQL, data integrity
  guarantees, SHA-256 compatibility between MySQL and PostgreSQL

Author: renecannao

README.md

@@ -11,6 +11,34 @@ See usage in the [MySQL docs](https://dev.mysql.com/doc/employee/en/index.html)
 [![CI PostgreSQL](https://github.com/datacharmer/test_db/actions/workflows/ci-postgresql.yml/badge.svg)](https://github.com/datacharmer/test_db/actions/workflows/ci-postgresql.yml)
 
 
+## Supported Versions
+
+This database is regularly tested against the following server versions:
+
+| Vendor | Versions |
+|--------|----------|
+| MySQL | 8.0, 8.4, 9.0, 9.2, 9.5, 9.6 |
+| Percona Server | 8.0, 8.4 |
+| MariaDB | 10.11, 11.4, 12.1 |
+| PostgreSQL | 16, 17 |
+
+All versions are tested weekly via CI using [ProxySQL/dbdeployer](https://github.com/ProxySQL/dbdeployer).
+
+### MySQL 9.x Notes
+
+Starting with MySQL 9.5, the `SOURCE` command requires the `--commands` flag on the client:
+
+    mysql --commands < employees.sql
+
+Starting with MySQL 9.6, the `MD5()` and `SHA()` functions have been removed from the server.
+The integrity test files `test_employees_md5.sql` and `test_employees_sha.sql` will not work on 9.6+.
+Use `test_employees_sha2.sql` instead, which uses `SHA2(..., 256)` and is compatible with all versions:
+
+    mysql -t < test_employees_sha2.sql
+
+The SHA-256 checksums are identical across all supported MySQL, Percona, MariaDB, and PostgreSQL versions.
+
+
 ## Where it comes from
 
 The original data was created by Fusheng Wang and Carlo Zaniolo at 
@@ -59,11 +87,11 @@ If you want to install with two large partitioned tables, run
 
 ## Testing the installation
 
-After installing, you can run one of the following
+After installing, you can run one of the following integrity tests:
 
-    mysql -t < test_employees_md5.sql
-    # OR
-    mysql -t < test_employees_sha.sql
+    mysql -t < test_employees_sha2.sql   # SHA-256 (works on all versions including 9.6+)
+    mysql -t < test_employees_md5.sql    # MD5 (MySQL 8.0–9.5 only)
+    mysql -t < test_employees_sha.sql    # SHA-1 (MySQL 8.0–9.5 only)
 
 For example:
 
@@ -107,7 +135,25 @@ For example:
 
 ## PostgreSQL Installation
 
-The database is also available for PostgreSQL. The schema and data are identical.
+The database is also available for PostgreSQL 12+. The schema and data are identical
+to the MySQL version. All files are in the `postgresql/` directory.
+
+### Differences from the MySQL version
+
+- **ENUM type**: MySQL `ENUM('M','F')` is replaced with `CHAR(1) CHECK (gender IN ('M','F'))`
+- **Stored procedures**: MySQL's `delimiter //` syntax is replaced with PostgreSQL dollar-quoting (`$...$ LANGUAGE plpgsql`)
+- **`show_departments()`**: Implemented as a function returning TABLE (use `SELECT * FROM show_departments();` instead of `CALL show_departments();`)
+- **User variables**: MySQL's `@var := value` pattern is replaced with PL/pgSQL local variables
+- **Integrity tests**: Use the same incremental hashing approach but via PL/pgSQL helper functions instead of MySQL user variables
+
+### Data integrity across databases
+
+The SHA-256 checksums are **identical** between MySQL and PostgreSQL. This is verified in CI:
+the same expected values in `test_employees_sha2.sql` and `postgresql/test_employees_sha2.sql`
+produce matching results on both databases. The MySQL version uses `SHA2(..., 256)` while
+PostgreSQL uses `encode(digest(..., 'sha256'), 'hex')` from the `pgcrypto` extension.
+
+### Installation
 
 1. Download the repository
 2. Install PostgreSQL (12+)
@@ -122,15 +168,16 @@ The database is also available for PostgreSQL. The schema and data are identical
 
 ### Testing the PostgreSQL installation
 
-        psql -d employees < postgresql/test_employees_md5.sql
-        # OR
-        psql -d employees < postgresql/test_employees_sha.sql
+        psql -d employees < postgresql/test_employees_sha2.sql   # SHA-256 (recommended)
+        psql -d employees < postgresql/test_employees_md5.sql    # MD5
+        psql -d employees < postgresql/test_employees_sha.sql    # SHA-1 (requires pgcrypto)
 
 ### Optional: load stored procedures and functions
 
         psql -d employees < postgresql/objects.sql
 
-Available functions: `emp_name()`, `emp_dept_name()`, `emp_dept_id()`, `current_manager()`, `show_departments()` (use `SELECT * FROM show_departments();`), `employees_help()`.
+Available functions: `emp_name()`, `emp_dept_name()`, `emp_dept_id()`, `current_manager()`,
+`show_departments()` (use `SELECT * FROM show_departments();`), `employees_help()`.
 
 
 ## DISCLAIMER