mysql字段拼接、查询重复数据、删除重复数据、还原字段默认值

mysql字段拼接、查询重复数据、删除重复数据、还原字段默认值

1.查询条件判断：

case when 条件 then 结果1（条件成立） else 结果2

2.查询分组排序：

row_number() over()，执行晚于 where 、group by
row_number() over(partition by 分组列 order by 排序列 desc)

group by 和 order by 同时使用：
order by 必须包含在 group by 中

3.字段拼接：name前面拼接一个1

update student set name = CONCAT('1',name)

4.还原字段默认值

update member set member_nick_name = DEFAULT(member_nick_name), member_true_name = DEFAULT("张三") where id = 1;

5.查询表中重复数据：

SELECT
   name
FROM
   student 
WHERE
   age = 15
GROUP BY
   name
HAVING
   count(name)> 1;

6.删除表中重复数据：

DELETE
FROM
   student 
WHERE
   id NOT IN ( SELECT id FROM ( SELECT min( id ) FROM student WHERE name = '张三' GROUP BY age ) A );

7.多个单条件查询
choose-when-ortherwise，相当于switch-case-default

<select id="selByCondition2" resultMap="rm">
    select *
    from mybatis where
    <choose>
        <when test="status !=null">
            STATUS=#{STATUS}
        </when>
        <when test="companyName !=null and companyName !=''">
            company_name like #{companyName}
        </when>
        <when test="bracdName !=null and bracdName !=''">
            bracd_name like #{bracdName}
        </when>
        <otherwise>1=1</otherwise>
    </choose>
</select