触发与更新

Question

我有以下结构的表“使用”后2个表：

Job | Start Time | End Time | Session 

而且我还有一个表“会议”具有下列结构来跟踪会话：

Session Label | Start Time | End Time 

我想有一个触发器，当第一个表中有条目时，触发器将检查其他表的开始时间和结束时间。如果已存在标签，则第一个表的会话列需要使用会话标签进行更新，否则应将新条目添加到第二个表中，并将相应的标签输入到第二个表的会话列中。我以前从来没有使用SQL触发器，也不知道如何实现这一点。

我想：

CREATE TRIGGER test AFTER INSERT ON `usage` 
FOR EACH ROW 
WHEN (SELECT RIGHT(`usage`.starttime,5) != SELECT RIGHT(`sessions`.starttime,5)) 
BEGIN 
    SET `usage`.sessionlabel = `A` 
END; 

逻辑：

for(newrecordinusagetable) 
{ 
//check if same starttime exists in any record of session table 
if(true) 
{ 
//do nothing 
} 
else 
{ 
//add the new starttime to the session table 
} 

PS：不要理会结束时间或标签。

Answer 1

下面是基于有限的信息提供了一个尝试：

DELIMITER $$ 

CREATE TRIGGER usage_session_trigger 
AFTER INSERT ON `usage` 
FOR EACH ROW 
    BEGIN 
     -- Count sessions with same start time 
     DECLARE session_count INT; 
     SET session_count = (SELECT COUNT(*) 
          FROM sessions 
          WHERE RIGHT(starttime, 5) = RIGHT(NEW.starttime, 5)); 

     -- If none found, insert one. 
     IF (session_count = 0) THEN 
      INSERT INTO sessions (sessionlabel, starttime, endtime) 
      VALUES (NEW.`session`, NEW.starttime, NEW.endtime); 
     END IF; 
    END; 
$$ 

DELIMITER ; 

注意

1. 我假设你有充分的理由时代的最后5个字符比较的例子等有在这里重复。 （不知道为什么5虽然！）
2. 也可能需要考虑是否需要类似的触发更新（+可能删除？）
3. 我不是在评论你是否要求做的是正确的做法 - 试图回答这个问题。

Answer 2

从更好的解决方案角度来看，最好在应用程序逻辑中将数据插入到使用表中时执行此操作，而不是创建触发器来实现此目的。为了演示你的风格，我使用了一个存储过程来插入使用表。您可以相应地修改它。

-- create usage table 
create table `usage` (job varchar(100), 
       start_time date, 
       end_time date, 
       `session` varchar(100) 
      ); 

-- create `sessions` table 
create table `sessions` (session_label varchar(100) , 
       start_time date, 
       end_time date); 

-- create procedure prc_insert_into_usage 
delimiter $$ 

Create Procedure prc_insert_into_usage 
(
    IN p_job varchar(100), 
    IN p_start_time date, 
    IN p_end_time date 
) 
Begin 
    Declare v_session varchar(100); 

    -- check if record is there in the session table 
    -- it is assumed session table will have only one record for composite key (start_time, end_time) 
    Select session_label into v_session 
    from `sessions` 
    where start_time = p_start_time and end_time = p_end_time; 

    IF (v_session IS NULL) THEN 

    -- if session_label is generated using auto increment key in sessions table then 
    -- last_insert_id() can be used to fetch that value after insert into sessions table 
    -- which can be used while inserting into usage table 
    -- example below 
    -- Insert into `sessions` (start_time,end_time) values (p_start_time,p_end_time); 
    -- set @v_session = last_insert_id(); 
    -- Insert into `usage` values (p_job,p_start_time,p_end_time,@v_session); 

    -- dummy logic to create the session_label 
    -- dummy logic is used you can replace it with whatever logic you need 
    set @v_session = concat('sess_',left(uuid(),8)); 

    -- insert record in both table (first in session table and then in usage table) 
    Insert into `sessions` values (@v_session,p_start_time,p_end_time); 
    Insert into `usage` values (p_job,p_start_time,p_end_time,@v_session); 

    else 
    -- record already present in sessions table for this session label 
    -- so insert record only in usage table 
    Insert into `usage` values (p_job,p_start_time,p_end_time,@v_session); 

    End If; 

End; 

$$ 

-- call this procedure to insert data into usage table 
-- sample execution below when both tables have no rows 
-- below call will insert 1 rows in both table having same session label 
mysql> call prc_insert_into_usage('job_a','2015-01-01','2015-01-02'); 
Query OK, 1 row affected (0.00 sec) 

mysql> select * from `usage`; 
+-------+------------+------------+---------------+ 
| job | start_time | end_time | session  | 
+-------+------------+------------+---------------+ 
| job_a | 2015-01-01 | 2015-01-02 | sess_abc376bf | 
+-------+------------+------------+---------------+ 
1 row in set (0.00 sec) 

mysql> select * from `sessions`; 
+---------------+------------+------------+ 
| session_label | start_time | end_time | 
+---------------+------------+------------+ 
| sess_abc376bf | 2015-01-01 | 2015-01-02 | 
+---------------+------------+------------+ 
1 row in set (0.01 sec) 

-- below call will insert only in usage table as row already present in sessions table 
mysql> call prc_insert_into_usage('job_b','2015-01-01','2015-01-02'); 
Query OK, 1 row affected (0.00 sec) 

mysql> select * from `usage`; 
+-------+------------+------------+---------------+ 
| job | start_time | end_time | session  | 
+-------+------------+------------+---------------+ 
| job_a | 2015-01-01 | 2015-01-02 | sess_abc376bf | 
| job_b | 2015-01-01 | 2015-01-02 | sess_abc376bf | 
+-------+------------+------------+---------------+ 
2 rows in set (0.00 sec) 

mysql> select * from `sessions`; 
+---------------+------------+------------+ 
| session_label | start_time | end_time | 
+---------------+------------+------------+ 
| sess_abc376bf | 2015-01-01 | 2015-01-02 | 
+---------------+------------+------------+ 
1 row in set (0.00 sec) 

-- below call will again insert rows in both table 
mysql> call prc_insert_into_usage('job_c','2015-01-02','2015-01-04'); 
Query OK, 1 row affected (0.01 sec) 

mysql> select * from `usage`; 
+-------+------------+------------+---------------+ 
| job | start_time | end_time | session  | 
+-------+------------+------------+---------------+ 
| job_a | 2015-01-01 | 2015-01-02 | sess_abc376bf | 
| job_b | 2015-01-01 | 2015-01-02 | sess_abc376bf | 
| job_c | 2015-01-02 | 2015-01-04 | sess_dcfa6853 | 
+-------+------------+------------+---------------+ 
3 rows in set (0.00 sec) 

mysql> select * from `sessions`; 
+---------------+------------+------------+ 
| session_label | start_time | end_time | 
+---------------+------------+------------+ 
| sess_abc376bf | 2015-01-01 | 2015-01-02 | 
| sess_dcfa6853 | 2015-01-02 | 2015-01-04 | 
+---------------+------------+------------+ 
2 rows in set (0.00 sec) 

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