left join t_alarm_rule tar on IF(tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = OBJECT_TYPE)

这是一条 SQL 查询语句中的左连接（left join）操作，其中 t_alarm_rule 表被命名为 tar， t1 表和该查询语句中的另一个未知表格在其它部分中被引用。在连接条件中，使用 IF 函数对 tar 表中的 ALARM_OBJECT_TYPE 进行条件判断，如果它等于 t1 表中的 DEVICE_ID，则连接条件为 tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID，否则为 tar.ALARM_OBJECT_TYPE = OBJECT_TYPE。这将返回一个包含 t1 表和 tar 表中匹配记录的结果集，其中 tar 表中的记录可能为空（因为是左连接）。

相关问题

select COLLECT_TIME, PERF_VALUE, t1.unit, RULEA, RULEB, RULEC from ( select COLLECT_TIME, PERF_VALUE, UNIT as unit, EQP_OBJ_ID, OBJECT_TYPE, INDEX_TYPE, DEVICE_ID, INDEX_NAME from t_perf_sensor_history tpsh, t_perf_defined tpd where PERF_OBJ_ID = tpd.OBJ_ID ) as t1 left join t_alarm_rule tar on IF(tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = OBJECT_TYPE) where 1=1 and COLLECT_TIME BETWEEN DATE_SUB(NOW(), INTERVAL 53 DAY) AND NOW() union select COLLECT_TIME, PERF_VALUE, t1.unit, RULEA, RULEB, RULEC from ( select COLLECT_TIME, PERF_VALUE, UNIT as unit, EQP_OBJ_ID, OBJECT_TYPE, INDEX_TYPE, DEVICE_ID, INDEX_NAME from t_perf_sensor_run tpsr, t_perf_defined tpd where PERF_OBJ_ID = tpd.OBJ_ID ) as t1 left join t_alarm_rule tar on IF(tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = OBJECT_TYPE) where 1=1 and COLLECT_TIME BETWEEN DATE_SUB(NOW(), INTERVAL 53 DAY) AND NOW() order by COLLECT_TIME desc怎么优化速度

首先，可以优化查询语句中的子查询，将其转换为 JOIN。例如：

SELECT COLLECT_TIME, PERF_VALUE, tpd.UNIT AS unit, t1.EQP_OBJ_ID, t1.OBJECT_TYPE, t1.INDEX_TYPE, t1.DEVICE_ID, t1.INDEX_NAME, tar.RULEA, tar.RULEB, tar.RULEC 
FROM ( 
    SELECT tpsh.COLLECT_TIME, tpsh.PERF_VALUE, tpsh.EQP_OBJ_ID, tpsh.OBJECT_TYPE, tpsh.INDEX_TYPE, tpsh.DEVICE_ID, tpsh.INDEX_NAME 
    FROM t_perf_sensor_history tpsh 
    JOIN t_perf_defined tpd ON tpsh.PERF_OBJ_ID = tpd.OBJ_ID 
    WHERE tpsh.COLLECT_TIME BETWEEN DATE_SUB(NOW(), INTERVAL 53 DAY) AND NOW() 
    
    UNION ALL 
    
    SELECT tpsr.COLLECT_TIME, tpsr.PERF_VALUE, tpsr.EQP_OBJ_ID, tpsr.OBJECT_TYPE, tpsr.INDEX_TYPE, tpsr.DEVICE_ID, tpsr.INDEX_NAME 
    FROM t_perf_sensor_run tpsr 
    JOIN t_perf_defined tpd ON tpsr.PERF_OBJ_ID = tpd.OBJ_ID 
    WHERE tpsr.COLLECT_TIME BETWEEN DATE_SUB(NOW(), INTERVAL 53 DAY) AND NOW() 
) t1 
LEFT JOIN t_alarm_rule tar ON (tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID OR tar.ALARM_OBJECT_TYPE = t1.OBJECT_TYPE) 
ORDER BY COLLECT_TIME DESC

这样可以避免重复子查询的问题，提高查询效率。

其次，可以考虑对表进行索引优化。在这个查询中，可以为 t_perf_sensor_history 表的 COLLECT_TIME 字段和 PERF_OBJ_ID 字段，以及 t_perf_sensor_run 表的 COLLECT_TIME 字段和 PERF_OBJ_ID 字段建立联合索引，以优化子查询的效率。

最后，可以考虑对 WHERE 子句中的日期范围限制进行优化。例如，可以根据业务需求调整日期范围的大小，避免查询过多无用数据。同时，可以考虑将日期范围限制转换为日期时间戳的形式，以避免对日期字符串的转换和比较，提高查询效率。

select DISTINCT COLLECT_TIME, PERF_VALUE, t1.unit, RULEA, RULEB, RULEC from ( select COLLECT_TIME, PERF_VALUE, UNIT as unit, EQP_OBJ_ID, OBJECT_TYPE, INDEX_TYPE, DEVICE_ID, INDEX_NAME from t_perf_sensor_history tpsh, t_perf_defined tpd where PERF_OBJ_ID = tpd.OBJ_ID ) as t1 left join t_alarm_rule tar on IF(tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = OBJECT_TYPE) where 1=1 and COLLECT_TIME BETWEEN DATE_SUB(NOW(), INTERVAL 62 DAY) AND now() order by COLLECT_TIME desc 怎末改才能避免重复数据

你可以在select子句中使用DISTINCT关键字去除重复数据，例如：

select DISTINCT COLLECT_TIME, PERF_VALUE, t1.unit, RULEA, RULEB, RULEC 
from ( 
    select COLLECT_TIME, PERF_VALUE, UNIT as unit, EQP_OBJ_ID, OBJECT_TYPE, INDEX_TYPE, DEVICE_ID, INDEX_NAME 
    from t_perf_sensor_history tpsh, t_perf_defined tpd 
    where PERF_OBJ_ID = tpd.OBJ_ID 
) as t1 
left join t_alarm_rule tar on IF(tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = OBJECT_TYPE) 
where 1=1 and COLLECT_TIME BETWEEN DATE_SUB(NOW(), INTERVAL 62 DAY) AND now() 
order by COLLECT_TIME desc

另外，你也可以使用 GROUP BY 子句来去除重复数据，例如：

select COLLECT_TIME, PERF_VALUE, t1.unit, RULEA, RULEB, RULEC 
from ( 
    select COLLECT_TIME, PERF_VALUE, UNIT as unit, EQP_OBJ_ID, OBJECT_TYPE, INDEX_TYPE, DEVICE_ID, INDEX_NAME 
    from t_perf_sensor_history tpsh, t_perf_defined tpd 
    where PERF_OBJ_ID = tpd.OBJ_ID 
) as t1 
left join t_alarm_rule tar on IF(tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = t1.DEVICE_ID, tar.ALARM_OBJECT_TYPE = OBJECT_TYPE) 
where 1=1 and COLLECT_TIME BETWEEN DATE_SUB(NOW(), INTERVAL 62 DAY) AND now() 
group by COLLECT_TIME, PERF_VALUE, t1.unit, RULEA, RULEB, RULEC 
order by COLLECT_TIME desc

注意，使用 GROUP BY 语句时需要将 select 子句中的每个非聚合列都包含在 GROUP BY 子句中。