In MySQL, the HAVING clause is used to filter the results after grouping, while WHERE is used to filter before grouping. 1. HAVING is used to conditionally filter the results of the aggregate function, such as SUM, AVG, COUNT, etc. For example, users who have a total consumption of more than 1,000 yuan need to use HAVING total > 1000; 2. WHERE filters the original data before grouping, such as filtering records with order amounts greater than 100 yuan; 3. Alias can be used after HAVING, but WHERE is not available; 4. HAVING can also use aggregate expressions directly, such as HAVING COUNT(*) > 5, no alias is required. The order of the two cannot be confused to ensure that the scenario is applied correctly to achieve the expected results.
The having clause is often used in mysql to filter the results after grouping. It works a bit like where, but it is used differently. where filters data before grouping, and having filters after grouping. For example, if you want to find some groups that do not meet certain conditions after you group by, you have to use having.
Here are a few actual scenarios to see how to use it:
When should I use having?
The most common usage scenario is when you need to filter the results of the aggregate function. For example, the values calculated by functions such as sum, avg, count, etc. cannot be filtered with where, because they are calculated after grouping. At this time, you need to have to appear.
For example: Suppose you have an order table orders that record the order amount of each user. If you want to find out the user who spends a total of more than 1,000 yuan, you can write it like this:
SELECT user_id, SUM(amount) AS total FROM orders GROUP BY user_id HAVING total > 1000;
The meaning of this statement is to first group by user_id, then calculate each person's total consumption, and finally only records with total consumption greater than 1,000 are retained.
What is the difference between having and where?
Simply put:
- where is the "pre-group" filtering data;
- having is the result of "after grouping".
For example, if you want to find records with order amounts greater than 100 and only keep users with total amounts greater than 500, you can write this way:
SELECT user_id, SUM(amount) AS total FROM orders WHERE amount > 100 GROUP BY user_id HAVING total > 500;
Here where filters a single record, and having filters the result after grouping.
Note: you can use an alias after having (such as total), but where cannot be done because that field has not been calculated yet.
Practical tips
Sometimes you will encounter the need to find users with less than 3 orders. How to write this time?
SELECT user_id, COUNT(*) AS num_orders FROM orders GROUP BY user_id HAVING num_orders < 3;
Here we use count(*) to count the order count of each user, and then use having to filter out records less than 3.
Another small detail is that you can also write expressions directly after having, such as:
HAVING COUNT(*) > 5
No alias is required, although it is slightly less readable, but it is very useful in some temporary queries.
Basically all this is it. The core of having is to filter the results after group by, especially conditional judgments that rely on aggregate functions. It may be easy to be confused with where at the beginning, so it will be clear if you write it a few more times.
The above is the detailed content of mysql having clause example. For more information, please follow other related articles on the PHP Chinese website!
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