在使用mybatis-plus进行条件查询时，遇到一个需求：使用一个输入框查询多个属性。乍一看这不挺简单的嘛，于是一通操作代码如下：

LambdaQueryWrapper<Student> queryWrapper = new LambdaQueryWrapper<Student>();
        queryWrapper.and(wrapper -> wrapper.like(Strings.isNotEmpty(student.getStudentName()),Student::getStudentName,student.getStudentName())
                        .or().like(Strings.isNotEmpty(student.getStudentName()),Student::getStudentNumber, student.getStudentName()));

然后结果尴尬了：

org.springframework.jdbc.BadSqlGrammarException: 
### Error querying 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 ') ORDER BY student_id ASC) TOTAL' at line 3
### The error may exist in com/chaopei/cxjw/mapper/StudentMapper.java (best guess)
### The error may involve defaultParameterMap
### The error occurred while setting parameters
### SQL: SELECT COUNT(*) FROM (SELECT  student_id,student_number,student_name,student_sex,faculty_id,major_id,class_id,grade_id,isdel  FROM student     WHERE () ORDER BY student_id ASC) TOTAL
### 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 ') ORDER BY student_id ASC) TOTAL' at line 3
; 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 ') ORDER BY student_id ASC) TOTAL' at line 3

但是去页面测试了一下发现虽然报错但是功能已经实现了，继续往上翻错误信息还有一段

 WHERE () ORDER BY student_id ASC", exception:
net.sf.jsqlparser.parser.ParseException: Encountered unexpected token: ")" ")"
    at line 3, column 9.

Was expecting one of:

    "!"
    "("
    "NOT"

百度了好多案例清一色都是说mybatis-plus的多租户过滤的问题，像下面给的方案，可是我mapper层只实现了mybatis-plus接口并没有写一个自己的方法，试了一大堆根本不管用。

 最后，静下心来仔细分析了错误信息，Encountered unexpected token: ")" ")"这不就是说多了两个括号的问题么，想想突然明白了，把拼接条件的判断语句写在里面，当条件为空时虽然逻辑上正确不会执行，但是拼接的语句（）却保留在了里面，而单个条件时就不会有这种情况，于是修改代码如下：

//多条件查询判断要写外面，写在里面会报错！！
        if(Strings.isNotEmpty(student.getStudentName())){
            queryWrapper.and(wrapper -> wrapper.like(Student::getStudentName,student.getStudentName())
                    .or().like(Student::getStudentNumber, student.getStudentName()));
        }

成功运行！！！

==>  Preparing: SELECT student_id,student_number,student_name,student_sex,faculty_id,major_id,class_id,grade_id,isdel FROM student WHERE ((student_name LIKE ? OR student_number LIKE ?) AND isdel = ?) ORDER BY student_id ASC LIMIT ?
==> Parameters: %测试%(String), %测试%(String), 0(Integer), 10(Long)
<==    Columns: student_id, student_number, student_name, student_sex, faculty_id, major_id, class_id, grade_id, isdel
<==        Row: 8, 20220006, 测试, 女, 1, 2, 2, 3, 0
<==        Row: 9, 20220007, 测试, 女, 1, 2, 1, 1, 0
<==        Row: 11, 20220009, 测试, 女, 1, 3, 2, 2, 0
<==      Total: 3

总结：条件判断一定要在外层做判断。欢迎分享哦~