Difference between on, where and having

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ON 、WHERE、HAVING Can filter data by limiting conditions , But their use is very different . Let's analyze the differences between the three .

1. ON and WHERE

All queries will generate an intermediate temporary report , The query result is obtained from the returned Temporary Report .ON and WHERE The difference between the following restrictions , It's mainly about the time when the constraints work ,ON Filter database records according to restrictions , Then produce a temporary table ; and WHERE After the production of the temporary watch , Filter the results from the temporary table according to the restrictions .

For the above reasons ,ON and WHERE The main differences are ：

1） Return results ： It's on the left （ Right outside ） Connecting ,ON Will return to the left table （ Right table ） All the records in ; and WHERE in , At this time, it is equivalent to inner join, Only records that meet the conditions will be returned （ Because it is filtered from the temporary table , Will filter out the ones that don't meet the conditions ）. 2） Speed ： because ON The constraints occurred earlier , The data set of the temporary table should be small , therefore ON Better than WHERE.

2. HAVING and WHERE

HAVING and WHERE The difference is also related to the time when the constraints work ,HAVING It is to filter the result after the result of aggregate function calculation , Query results only return qualified groups ,HAVING Not alone , Only now GROUP BY clause .; and WHERE It's filtering results before calculating , Suppose the aggregate function uses WHERE, Then the aggregate function only calculates that WHERE Clause constraints data , such as ：

     SELECT COUNT(id) FROM db_equip WHERE tb_equip_type = ‘2’;

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Count The result of the calculation is to first filter the device type as 2 My equipment , Then count the device type as 2 Number of types .

In use and function ,HAVING and WHERE There are the following differences ：

1） HAVING Not alone , Only now GROUP BY In clause ;WHERE That is, we can SELECT And other clauses , Can also be with GROUP BY Clause collocation ,WHERE The priority of aggregate function is higher than that of aggregate function HAVING. 2） because WHERE Filter data before aggregating functions ,HAVING Filter the grouping after the calculation , therefore WHERE It's faster than HAVING The query speed is fast .

3. summary

ON、WHERE、HAVING The main difference is that the restrictive condition in its clause is caused by the time when it works ,ON It is to filter records according to conditions before producing temporary tables ,WHERE It is to filter data from the temporary table of production , and HAVING It refers to the data that meets the conditions in the temporary table , After calculating and grouping , adopt HAVING Restrict statement filter grouping , The return result is that HAVING Grouping restricted by clause .

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