SELECT dmtr.`day`, dmtr.column_num, dmtr.table_num FROM dap_metadata_table_record AS dmtr GROUP BY dmtr.`day` ORDER BY dmtr.`day`

这是一个 SQL 查询语句，作用是从名为 `dap_metadata_table_record` 的表中查询出每个不同 `day` 值对应的 `column_num` 和 `table_num` 列，并按 `day` 值升序排序。具体解释如下：

- `SELECT dmtr.`day`, dmtr.column_num, dmtr.table_num`：指定查询的列，分别为 `day`、`column_num` 和 `table_num`。
- `FROM dap_metadata_table_record AS dmtr`：指定查询的表为 `dap_metadata_table_record`，并使用别名 `dmtr`。
- `GROUP BY dmtr.`day``：按 `day` 列进行分组，即将具有相同 `day` 值的记录看成一组。
- `ORDER BY dmtr.`day``：按 `day` 列的值升序排序，即先查询出最小的 `day` 值的记录，然后是次小的，依次类推。

需要注意的是，在 `SELECT` 语句中选择的列必须是在 `GROUP BY` 子句中出现的列，或者是使用聚合函数（如 `COUNT`、`SUM`）计算的列。否则，执行这个查询语句时会报错。

相关问题

SELECT dmtr.`day`, dmtr.column_num, dmtr.table_num FROM dap_metadata_table_record AS dmtr 将以上sql加上只查询day字段最大的7条数据

可以使用如下SQL语句来实现：

SELECT dmtr.`day`, dmtr.column_num, dmtr.table_num
FROM dap_metadata_table_record AS dmtr
WHERE dmtr.`day` IN (
    SELECT DISTINCT `day` FROM dap_metadata_table_record
    ORDER BY `day` DESC LIMIT 7
)
ORDER BY dmtr.`day` DESC

这条SQL语句会先查询出所有不重复的日期，并按日期降序排列，然后只选取前7条日期。接着，外层查询会根据这7条日期来筛选出对应的数据行，并再次按日期降序排列。最终结果是只包含day字段最大的7条数据。

SELECT DATE_FORMAT( dmtr.`day`,'%Y-%m-%d') as oneday, dmtr.column_num, dmtr.table_num FROM dap_metadata_table_record AS dmtr GROUP BY oneday HAVING oneday >= DATE_SUB(CURDATE(), INTERVAL 7 DAY) UNION ALL SELECT d.`day`, 0, 0 FROM ( SELECT DATE_SUB(CURDATE(), INTERVAL 7 DAY) AS day UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 6 DAY) UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 5 DAY) UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 4 DAY) UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 3 DAY) UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 2 DAY) UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 1 DAY) ) AS d LEFT JOIN dap_metadata_table_record AS dmtr1 ON d.`day` = dmtr1.`day` WHERE dmtr1.`day` IS NULL ORDER BY oneday DESC; 修改sql只查询七条

你可以将原来的查询语句中的 `HAVING` 子句改为 `WHERE` 子句，并添加一个条件限制只查询最近七天的记录，修改后的 SQL 查询语句如下：

SELECT DATE_FORMAT(dmtr.`day`, '%Y-%m-%d') AS oneday, dmtr.column_num, dmtr.table_num 
FROM dap_metadata_table_record AS dmtr 
WHERE dmtr.`day` >= DATE_SUB(CURDATE(), INTERVAL 7 DAY)
GROUP BY oneday
UNION ALL
SELECT d.`day`, 0, 0
FROM (
    SELECT DATE_SUB(CURDATE(), INTERVAL 7 DAY) AS day
    UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 6 DAY)
    UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 5 DAY)
    UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 4 DAY)
    UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 3 DAY)
    UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 2 DAY)
    UNION ALL SELECT DATE_SUB(CURDATE(), INTERVAL 1 DAY)
) AS d
LEFT JOIN dap_metadata_table_record AS dmtr1 ON d.`day` = dmtr1.`day`
WHERE dmtr1.`day` IS NULL
ORDER BY oneday DESC
LIMIT 7;

以上 SQL 查询语句中，`WHERE dmtr.`day` >= DATE_SUB(CURDATE(), INTERVAL 7 DAY)` 子句限制只查询最近七天的记录，`LIMIT 7` 子句限制只查询七条记录。