1.使用场景：在MySQL中，JSON作为一种数据类型存储在表的列中。需计算键值对的数量。
2.方法：SELECT COUNT(chief>'$.number') FROM t_projectapplication where id = #{id};（t_projectapplication：表；chief：包含JSON类型数据的列名；number：JSON数据中的某个键名）；
3.进阶使用场景：计算包含两个JSON类型数据字段之和；
4.方法：SELECT COUNT(cnt) FROM ( SELECT COUNT(chief>'$.number') AS cnt FROM t_projectapplication where id = #{id} UNION ALL SELECT COUNT(department_leader>'$.number') AS cnt FROM t_projectapplication where where id = #{id} ) AS countdata