SQL Case條件判斷語句

問題描述：在表中取到一些值做出判斷，配合監控監測一些表中的數據。使用select case when if 來做條件查詢判斷

 

CASE 表達式遍歷條件並在滿足第一個條件時返回一個值（類似於 if-then-else 語句）。 因此，一旦條件爲真，它將停止讀取並返回結果。 如果沒有條件爲真，則返回 ELSE 子句中的值。

如果沒有 ELSE 部分並且沒有條件爲真，則返回 NULL。

case when語法結構

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

 

 

 表結構：

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)

 數據：

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.備份表，通過判斷備份表中最後一次備份是否成功，或者失敗。成功返回success，失敗返回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;

如果最後一條數據的backup_process 字段數據爲success，sql返回結果就是'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)

如果最後一條數據的backup_process 字段數據爲其它，sql返回結果就是'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.備份表，查詢最後一條備份記錄後的一段時間段內，有沒有新的備份記錄產生，如果沒有，返回failed；如果備份記錄正常產生，返回success。如果新的備份記錄沒有在規定時間內被記錄，此時備份腳本狀態可能有異常。

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;

通過判斷最後一條記錄的備份時間跟now()此時的時間比較，判斷下一次的記錄有沒有在規定的時間產生，以此來判斷備份腳本的執行狀態
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)