[MySQL must know and know (9)] [Summary data]

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1. Aggregate function

Aggregate functions run on row groups, functions that calculate and return a single value

1.1 AVG() function

AVG() finds the average value of the column by counting the number of rows in the table and calculating the sum of the values ​​of a specific column

mysql> SELECT AVG(prod_price) AS avg_price
    -> FROM products;

AVG() can also be used to determine the average value of a specific column or row

mysql> SELECT AVG(prod_price) AS avg_price
    -> FROM products
    -> WHERE vend_id = 1003;

Only for a single column

AVG() can only be used to determine the average value of a specific numeric column, and the column name must be given as a function parameter. In order to obtain the average value of multiple columns, multiple AVG() functions must be used.

NULL value

AVG() function ignores rows whose column value is NULL

1.2 COUNT() function

COUNT() function to count

Returns the total number of customers in the customers table

mysql> SELECT COUNT(*) AS num_cust
    -> FROM customers;

Count customers with email addresses

mysql> SELECT COUNT(cust_email) AS num_cust
    -> FROM customers;

NULL value

If you specify a column name, the row whose value in the specified column is empty is ignored by the COUNT() function, but if the asterisk (*) is used in the COUNT() function, it is not ignored

1.3 MAX() function

mysql> SELECT MAX(prod_price) AS max_price
    -> FROM products;

Use MAX() for non-numeric data

Although MAX() is generally used to find the largest number or date value, MySQL allows it to be used to return the maximum value in any column, including the maximum value in a text column. When used for text data, if the data is sorted by the corresponding column, MAX() returns the last row

NULL value

MAX() function ignores rows whose column value is NULL

1.4 MIN() function

mysql> SELECT MIN(prod_price) AS min_price
    -> FROM products;

Use MIN() for non-numeric data

MySQL allows it to be used to return the minimum value in any column, including the minimum value in a text column. When used for text data, if the data is sorted by the corresponding column, MIN() returns the top row

NULL value

The MIN() function ignores rows whose column value is NULL

1.5 SUM() function

mysql> SELECT SUM(quantity) AS items_ordered
    -> FROM orderitems
    -> WHERE order_num = 20005;

SUM can also be used to total calculated values

mysql> SELECT SUM(item_price*quantity) AS total_price
    -> FROM orderitems
    -> WHERE order_num = 20005;

Calculate on multiple columns

Using standard arithmetic operators, all aggregate Hassan farmers can be used to perform calculations on multiple columns

NULL value
SUM() function ignores rows whose column value is NULL

Two, gather different values

ALL is the default

The ALL parameter does not need to be specified because it is the default behavior. If DISTINCT is not specified, it is assumed to be ALL

Use the AVG() function to return the average price of a product provided by a specific supplier

mysql> SELECT AVG(DISTINCT prod_price) AS avg_price
    -> FROM products
    -> WHERE vend_id = 1003;

note

DISTINCT must use column names and cannot be used in calculations or expressions

Three, combined aggregate function

mysql> SELECT COUNT(*) AS num_items,
    -> MIN(prod_price) AS price_min,
    -> MAX(prod_price) AS price_max,
    -> AVG(prod_price) AS price_avg
    -> FROM products;

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