[MySQL must know and know (ten)] [Group data]

Previous: [MySQL Must Know and Know (9)] [Summary Data]

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1. Data grouping

Returns the number of products provided by supplier 1003

mysql> SELECT COUNT(*) AS num_prods
    -> FROM products
    -> WHERE vend_id =1003;

Two, create a group

mysql> SELECT vend_id, COUNT(*) AS num_prods
    -> FROM products
    -> GROUP BY vend_id;

Regarding the provisions of GROUP BY 1. The
GROUP BY clause can contain any number of columns
. 2. Groups are nested in the GROUP BY clause, and the data will be summarized on the last specified group. 3. The GROUP BY clause is
listed in the sentence Each column of must be a retrieval column or a valid expression
4. Except for aggregate calculation statements, each column in the SELECT statement must be given in the GROUP BY clause
5. If the grouping has a NULL value, then NULL Will be returned as a group.
6. The GROUP BY clause must appear after the WHERE clause and before the ORDER BY clause

Three, filter group

HAVING supports all WHERE operators

The conditions and options of the WHERE clause are applicable to HAVING, the syntax is the same, the keywords are different

mysql> SELECT cust_id, COUNT(*) AS orders
    -> FROM orders
    -> GROUP BY cust_id
    -> HAVING COUNT(*) >= 2;

Difference between HAVING and WHERE

WHERE filters before data grouping, HAVING filters after data grouping

List suppliers with more than two products with a price of 10 or more

mysql> SELECT vend_id, COUNT(*) AS num_prods
    -> FROM products
    -> WHERE prod_price >= 10
    -> GROUP BY vend_id
    -> HAVING COUNT(*) >= 2;

mysql> SELECT vend_id,COUNT(*) AS num_prods
    -> FROM products
    -> GROUP BY vend_id
    -> HAVING COUNT(*) >= 2;

Four, grouping and sorting

Retrieve the order number and total order price of orders whose total order price is greater than or equal to 50

mysql> SELECT order_num, SUM(quantity*item_price) AS ordertotal
    -> FROM orderitems
    -> GROUP BY order_num
    -> HAVING SUM(quantity*item_price) >= 50;

Sort output by total order price

mysql> SELECT order_num, SUM(quantity*item_price) AS ordertotal
    -> FROM orderitems
    -> GROUP BY order_num
    -> HAVING SUM(quantity*item_price) >= 50
    -> ORDER BY ordertotal;

Five, SELECT clause order

SELECT-FROM-WHERE-GROUP BY (group description)-HAVING (group-level filtering)-ORDER BY-LIMIT

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