为什么MyBatis-Plus拼装的列表查询语句selectPage中带有WHERE子句

一般情况下，MyBatis-Plus拼装的列表查询语句是类似下面这样的：

SELECT id,name,gender,age,address,telephone,is_show FROM staff

但这次实测时，却在控制台输出了这样让人大跌眼镜的语句：

2021-01-07 10:57:34.822 DEBUG 36624 --- [nio-7000-exec-1] c.e.b.***.dao.xxxDao.selectPage  : ==>  Preparing: 
SELECT id,name,gender,age,address,telephone,is_show FROM staff WHERE is_show=1

另外，本来好好的UPDATE staff SET address=? WHERE id=?或UPDATE staff SET is_show=? WHERE id=?之类的语句，却一不留神被变成了下面的画风：

UPDATE staff SET address=? WHERE id=? AND is_show=1
UPDATE staff    WHERE id=? AND is_show=1 -- (这没有SET子句的UPDATE是什么鬼？——编者问)

经过反复多次实验，发现问题的症结所在，就是在application.yml中配置了逻辑删除字段。如下所示：

mybatis-plus:
  mapper-locations: classpath:/mapper/**/*.xml
  global-config:
    db-config:
      id-type: auto
      logic-delete-field: isShow  # 全局逻辑删除的实体字段名
      logic-delete-value: 0 # 逻辑已删除值(默认为 1)
      logic-not-delete-value: 1 # 逻辑未删除值(默认为 0)

------------------------------------------------始料未及的分割线---------------------------------------------------
如果需要设置逻辑删除字段，又不想出现上面诡异的SQL语句，可以参考我前面刚写的《SpringCloud整合MyBatis-Plus逻辑删除配置错误时可能导致的严重后果》中的“方案二”。

总之，如果逻辑删除字段设置不当，不仅有误删数据的可能，还可能会看到页面及后端出现的语意不清的异常，如：

1、前端控制台异常：
POST http://localhost:88/api/company/***/update 500 (Internal Server Error)
Uncaught (in promise) Error: Request failed with status code 500
    at createError (createError.js?16d0:16)
    at settle (settle.js?db52:18)
    at XMLHttpRequest.handleLoad (xhr.js?ec6c:77)
    
2、后端控制台异常：
Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; 
	nested exception is org.springframework.jdbc.BadSqlGrammarException: 
	### Error updating database.  Cause: java.sql.SQLSyntaxErrorException: You have an error in your SQL syntax; 
        check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE id=7  AND is_show=1' at line 1
	### The error may exist in com/xxx/xxxx/company/dao/***Dao.java (best guess)
	### The error may involve com.xxx.xxxx.company.dao.***Dao.updateById-Inline
	### The error occurred while setting parameters
	### SQL: UPDATE staff    WHERE id=? AND is_show=1
	### Cause: java.sql.SQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE id=7  AND is_show=1' at line 1;bad SQL grammar []; nested exception is java.sql.SQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE id=7  AND is_show=1' at line 1] with root cause

由于上面的问题原因比较隐蔽，影响巨大，所以，在MyBatis-Plus中使用逻辑删除字段时，需要三思而行！