SQL Case条件判断语句

问题描述：在表中取到一些值做出判断，配合监控监测一些表中的数据。使用select case when if 来做条件查询判断

CASE 表达式遍历条件并在满足第一个条件时返回一个值（类似于 if-then-else 语句）。 因此，一旦条件为真，它将停止读取并返回结果。 如果没有条件为真，则返回 ELSE 子句中的值。

如果没有 ELSE 部分并且没有条件为真，则返回 NULL。

case when语法结构

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    WHEN conditionN THEN resultN
    ELSE result
END;

表结构：

MariaDB [test]> desc backup_job_details;
+----------------+--------------+------+-----+---------+-------+
| Field          | Type         | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+-------+
| server_id      | int(11)      | YES  |     | NULL    |       |
| host_name      | varchar(20)  | YES  |     | NULL    |       |
| host_ip        | varchar(50)  | YES  |     | NULL    |       |
| backup_type    | char(20)     | YES  |     | NULL    |       |
| start_time     | datetime     | YES  |     | NULL    |       |
| end_time       | datetime     | YES  |     | NULL    |       |
| backup_process | char(20)     | YES  |     | NULL    |       |
| backup_size    | char(20)     | YES  |     | NULL    |       |
| backup_dir     | varchar(200) | YES  |     | NULL    |       |
+----------------+--------------+------+-----+---------+-------+
9 rows in set (0.002 sec)

数据：

MariaDB [test]> select * from backup_job_details;
+-----------+-----------------+---------------+-------------+---------------------+---------------------+----------------+-------------+-----------------------------------------------------------------------+
| server_id | host_name       | host_ip       | backup_type | start_time          | end_time            | backup_process | backup_size | backup_dir                                                            |
+-----------+-----------------+---------------+-------------+---------------------+---------------------+----------------+-------------+-----------------------------------------------------------------------+
|        54 | Test-MySQL-DB03 | 192.168.163.21 | inc         | 2023-01-29 12:10:01 | 2023-01-29 12:10:04 | success        | 21M         | /localbackup/backup_mariadb/Inc_Test-MySQL-DB03-2023-01-29_12-10.zip  |
|        54 | Test-MySQL-DB03 | 192.168.163.21 | full        | 2023-01-30 00:20:01 | 2023-01-30 00:20:03 | success        | 41M         | /localbackup/backup_mariadb/Full_Test-MySQL-DB03-2023-01-30_00-20.zip |
|        54 | Test-MySQL-DB03 | 192.168.163.21 | inc         | 2023-01-30 12:10:01 | 2023-01-30 12:10:04 | success        | 21M         | /localbackup/backup_mariadb/Inc_Test-MySQL-DB03-2023-01-30_12-10.zip  |
|        54 | Test-MySQL-DB03 | 192.168.163.21 | full        | 2023-01-31 00:20:01 | 2023-01-31 00:20:05 | success        | 41M         | /localbackup/backup_mariadb/Full_Test-MySQL-DB03-2023-01-31_00-20.zip |
|        54 | Test-MySQL-DB03 | 192.168.163.21 | inc         | 2023-01-31 12:10:01 | 2023-01-31 12:10:04 | success        | 21M         | /localbackup/backup_mariadb/Inc_Test-MySQL-DB03-2023-01-31_12-10.zip  |
|        54 | Test-MySQL-DB03 | 192.168.163.21 | full        | 2023-02-01 00:20:01 | 2023-02-01 00:20:03 | success        | 41M         | /localbackup/backup_mariadb/Full_Test-MySQL-DB03-2023-02-01_00-20.zip |
|        54 | Test-MySQL-DB03 | 192.168.163.21 | inc         | 2023-02-01 12:10:01 | 2023-02-01 12:10:04 | success        | 21M         | /localbackup/backup_mariadb/Inc_Test-MySQL-DB03-2023-02-01_12-10.zip  |
|        54 | Test-MySQL-DB03 | 192.168.163.21 | inc         | 2023-02-01 12:10:01 | 2023-02-01 12:10:04 | failed         | 0M          |                                                                       |
|        54 | Test-MySQL-DB03 | 192.168.163.21 | inc         | 2023-02-01 12:10:01 | 2023-02-01 12:10:04 | failed         | 0M          |                                                                       |
|        54 | Test-MySQL-DB03 | 192.168.163.21 | inc         | 2023-02-01 12:10:01 | 2023-02-01 12:10:04 | success        | 10M         

1.备份表，通过判断备份表中最后一次备份是否成功，或者失败。成功返回success，失败返回failed

select case when backup_process = 'success' then 'success' when backup_process != 'success' then 'failed' end as backup_process from test.backup_job_details order by server_id DESC limit 1;

如果最后一条数据的backup_process 字段数据为success，sql返回结果就是'success'

MariaDB [(none)]> select case when backup_process = 'success' then 'success' when backup_process != 'success' then 'failed' end as backup_process from test.backup_job_details order by server_id DESC limit 1;
+----------------+
| backup_process |
+----------------+
| success        |
+----------------+
1 row in set (0.001 sec)

如果最后一条数据的backup_process 字段数据为其它，sql返回结果就是'failed'

MariaDB [(none)]> select case when backup_process = 'success' then 'success' when backup_process != 'success' then 'failed' end as backup_process from test.backup_job_details order by server_id DESC limit 1;
+----------------+
| backup_process |
+----------------+
| failed        |
+----------------+
1 row in set (0.001 sec)

2.备份表，查询最后一条备份记录后的一段时间段内，有没有新的备份记录产生，如果没有，返回failed；如果备份记录正常产生，返回success。如果新的备份记录没有在规定时间内被记录，此时备份脚本状态可能有异常。

SELECT (CASE WHEN TIMESTAMPDIFF(HOUR,end_time, now()) > 14 THEN "failed" ELSE "success" END )AS TIME_DIFF FROM test.backup_job_details order by end_time DESC limit 1;

通过判断最后一条记录的备份时间跟now()此时的时间比较，判断下一次的记录有没有在规定的时间产生，以此来判断备份脚本的执行状态
MariaDB [(none)]> SELECT (CASE WHEN TIMESTAMPDIFF(HOUR,end_time, now()) > 14 THEN "failed" ELSE "success" END )AS TIME_DIFF FROM test.backup_job_details order by end_time DESC limit 1;
+-----------+
| TIME_DIFF |
+-----------+
| success   |
+-----------+
1 row in set (0.001 sec)