Added MySQL 8 example

Author: mfvanek

mysql/README.md

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+# Run
+```shell
+docker-compose up -d
+```
+
+# Stop
+```shell
+docker-compose down
+```
+
+# Logs
+```shell
+docker compose logs 
+```
+
+# Connection string
+`jdbc:mysql://localhost:3306/sandbox`
+
+# Query
+```sql
+SELECT `course_name`, ROUND(SUM(cnt) / COUNT(`month`), 2) `avg_months`
+FROM (SELECT `course_name`, month (`subscription_date`) `month`, COUNT(*) cnt
+      FROM `PurchaseList`
+      WHERE YEAR (`subscription_date`) = 2018
+      GROUP by `course_name`, month (`subscription_date`)) t
+GROUP by `course_name`
+ORDER BY `avg_months` desc
+```

mysql/docker-compose.yml

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+version: '3'
+
+services:
+  db:
+    container_name: mysql_8
+    restart: always
+    image: mysql:8.0.35-oracle
+    command: mysqld --default-authentication-plugin=mysql_native_password --character-set-server=utf8 --collation-server=utf8_unicode_ci
+    environment:
+      MYSQL_ROOT_PASSWORD: root
+      MYSQL_DATABASE: sandbox
+#      MYSQL_USER: sandbox_user
+#      MYSQL_PASSWORD: passpass
+#      MYSQL_ALLOW_EMPTY_PASSWORD: "yes"
+    ports:
+      - '3306:3306'
+    volumes:
+      - './docker/db/data:/var/lib/mysql'
+#      - './docker/db/my.cnf:/etc/mysql/conf.d/my.cnf'
+      - './docker/db/sql:/docker-entrypoint-initdb.d'