Aggregate data-aggregate function

1. Aggregate function

We often need to summarize data without actually retrieving them. To facilitate this type of retrieval, MySQL provides five aggregate functions, as follows:

---

1.1 AVG() function

The avg() function ignores rows whose column value is null.

---

1.2 count() function

The count() function counts, there are two ways to use it:

1.count(*) counts the number of rows in the table, regardless of whether the table column contains a null value (null) or a non-null value.
2. count(column) counts the rows with values ​​in a specific column, ignoring
null values.

---

1.3 max() function

The max() function ignores rows whose column value is null.

---

1.4 min() function

The min() function ignores rows whose column value is null.

---

1.5 sum() function

The sum() function ignores rows whose column value is null.

---

2. Gather different values

---

3. Combining aggregate functions

---

-- AVG()函数
SELECT AVG(prod_price) AS avg_price FROM products;

-- AVG()函数，确定特定列的值
SELECT AVG(prod_price) AS avg_price FROM products WHERE vend_id = 1003;

-- count(*)函数，对所有行计数
SELECT COUNT(*) AS num_cust FROM customers;

-- count(列名)，只对列中有值的进行计数,不包含null值
SELECT COUNT(cust_email) AS num_cust FROM customers;

-- max()函数
SELECT MAX(prod_price) AS max_price FROM products;

-- min()函数
SELECT MIN(prod_price) AS min_price FROM products;

-- sum()函数
SELECT SUM(quantity) AS items_ordered FROM orderitems WHERE order_num = 20005;

-- SUM()也可以用来合计计算值
SELECT SUM(item_price*quantity) AS total_price FROM orderitems WHERE order_num = 20005;

-- distinct()函数，只包含不同的值
SELECT AVG(DISTINCT prod_price) AS avg_price FROM products WHERE vend_id = 1003;

-- 组合聚集函数，多个聚集函数组成
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;