Mybatis-Plus中and()和or()的使用与原理详解

一. 简单无优先级连接(即无括号的sql语句)

1. 与连接 and()

2. 或连接 or()

二. 复杂有优先级的的连接

补充：MybatisPlus中and和or的组合使用

总结

一. 简单无优先级连接(即无括号的sql语句)

简单来说,两个子条件间默认and与连接,若两个之间显式写出or()则or或连接.

1. 与连接 and()

当需要简单的将两个条件与连接,则最直接的写法为:

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

当然也可以显式地写出and()如下,但没必要:

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

2. 或连接 or()

当需要简单的将两个条件或连接,则最直接的写法为:

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

当然也可以如下,但不那么直观:

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

二. 复杂有优先级的的连接

上面有2个不推荐的做法,是因为sql语句为A or B , A and B这种简单连接.当涉及到诸如 A and ( B or C) and D 这类的复杂有优先级的的连接,直接拼接会导致成为 A and B or C and D.所以这时候需要需要or(Consumer consumer),and(Consumer consumer)这两个方法.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语句如下:

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

由此还可见or(Consumer consumer),and(Consumer consumer)这两个方法参数为Consumer时,会在连接处生成2对括号,以此提高优先级.

补充：MybatisPlus中and和or的组合使用

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();

总结

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