Mybatis-Plus中and()和or()的使用與原理詳解

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一. 簡單無優先級連接(即無括號的sql語句)

簡單來說,兩個子條件間默認and與連接,若兩個之間顯式寫出or()則or或連接.

1. 與連接 and()

當需要簡單的將兩個條件與連接,則最直接的寫法為:

QueryWrapper<AttrEntity> queryWrapper = new QueryWrapper<AttrEntity>().
eq("attr_id",key).
eq("catelog_id",catelogId);

當然也可以顯式地寫出and()如下,但沒必要:

QueryWrapper<AttrEntity> queryWrapper = new QueryWrapper<AttrEntity>().
eq("attr_id",key);
 
queryWrapper.and(qr -> qr.eq("catelog_id", catelogId));

2. 或連接 or()

當需要簡單的將兩個條件或連接,則最直接的寫法為:

QueryWrapper<AttrEntity> queryWrapper = new QueryWrapper<AttrEntity>().
eq("attr_id",key).
or().
eq("catelog_id",catelogId);

當然也可以如下,但不那麼直觀:

QueryWrapper<AttrEntity> queryWrapper = new QueryWrapper<AttrEntity>().
eq("attr_id",key);
 
queryWrapper.or(qr -> qr.eq("catelog_id", catelogId));

二. 復雜有優先級的的連接

上面有2個不推薦的做法,是因為sql語句為A or B , A and B這種簡單連接.當涉及到諸如 A and ( B or C) and D 這類的復雜有優先級的的連接,直接拼接會導致成為 A and B or C and D.所以這時候需要需要or(Consumer consumer),and(Consumer consumer)這兩個方法.示例如下:

        QueryWrapper<AttrEntity> queryWrapper = new QueryWrapper<AttrEntity>().eq("attr_type", "base".equalsIgnoreCase(type) ? 1 : 0);
        queryWrapper.and(qr -> 
                qr.eq("attr_id", key).
                   or().
                   like("attr_name", key)
        );
        queryWrapper.and(qr -> qr.eq("catelog_id", catelogId));

生成的sql語句如下:

select ...
WHERE (attr_type = ? AND ( (attr_id = ? OR attr_name LIKE ?) ) AND ( (catelog_id = ?) ))
...;

由此還可見or(Consumer consumer),and(Consumer consumer)這兩個方法參數為Consumer時,會在連接處生成2對括號,以此提高優先級.

補充：MybatisPlus中and和or的組合使用

案例1：where   A=? and B=? 

        //SELECT id,name,age,sex FROM student WHERE (name = ? AND age = ?)
        List<Student> list = studentService.lambdaQuery().eq(Student::getName, "1").eq(Student::getAge, 1).list();

案例2：where A=? or B=? 

        //SELECT id,name,age,sex FROM student WHERE (name = ? OR age = ?)
        List<Student> list = studentService.lambdaQuery().eq(Student::getName, "1").or().eq(Student::getAge, 12).list();

案例3：where A=? or（C=? and D=?) 

        //SELECT id,name,age,sex FROM student WHERE (name = ? OR (name = ? AND age = ?)) 
      List<Student> list =
          studentService
              .lambdaQuery()
              .eq(Student::getName, "1")
              .or(wp -> wp.eq(Student::getName, "1").eq(Student::getAge, 12))
              .list();

案例4：where (A=?andB=?)or(C=?andD=?) 

    // SELECT id,name,age,sex FROM student WHERE ((name = ? AND age = ?) OR (name = ? AND age = ?)) 
    List<Student> list =
        studentService
            .lambdaQuery()
            .and(wp -> wp.eq(Student::getName, "1").eq(Student::getAge, 12))
            .or(wp -> wp.eq(Student::getName, "1").eq(Student::getAge, 12))
            .list();

案例5：whert  A =? or (B=? and ( C=? or D=?)) 

    // SELECT * FROM student WHERE ((name <> 1) OR (name = 1 AND (age IS NULL OR age >= 11)))
    List<Student> list =
        studentService
            .lambdaQuery()
            .and(wp -> wp.ne(Student::getName, "1"))
            .or(
                wp ->
                    wp.eq(Student::getName, "1")
                        .and(wpp -> wpp.isNull(Student::getAge).or().ge(Student::getAge, 11)))
            .list();

總結

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