group by 和 distinct 的区别

SELECT
    fs.card_id,
    fs. NAME,
    fs.email,
    fs.phone_num,
    fs.weixin_num,
    fs.permission,
    fs.open_id
FROM
    fs_card fs
WHERE
    fs.open_id IS NOT NULL
GROUP BY fs.name

//group by  是 按照 name 进行分组 ，  结果显示名字不重复的一组数据。如果存在两条名字相同的数据，则只取出第一条数据。

SELECT
    fs.card_id,
    fs. NAME,
    fs.email,
    fs.phone_num,
    fs.weixin_num,
    fs.permission,
    fs.open_id
FROM
    fs_card fs
WHERE
    fs.open_id IS NOT NULL
GROUP BY fs.name having COUNT(fs.card_id)= 1

//加上count (fs.card_id)=1 条件后会把 具有相同名字的数据 去除（默认这两条数据的card_id 不同） 

SELECT
    fs.card_id,
    fs. NAME,
    fs.email,
    fs.phone_num,
    fs.weixin_num,
    fs.permission,
    fs.open_id
FROM
    fs_card fs
WHERE
    fs.open_id IS NOT NULL
GROUP BY fs.name having COUNT(fs.card_id)>= 1

//查询结果和 不加 having 条件 相同。

SELECT distinct
    fs.card_id,
    fs. NAME,
    fs.email,
    fs.phone_num,
    fs.weixin_num,
    fs.permission,
    fs.open_id
FROM
    fs_card fs
WHERE
    fs.open_id IS NOT NULL
GROUP BY fs.name having COUNT(fs.card_id)= 1

//distinct 的作用域是   所有的查询字段， 是对 该条查询结果数据 进行 去重复（比较的是结果集，即每条记录），而不是对某个字段。如果上面的名字相同的两条数据如果有查询的某个字段不同，则按照不同结果处理