Database learning notes II

Query statement expansion

limit Use （mysql Medium limit Be similar to sqlserver Medium top）

Use case table Student

The first one is ：SELECT * FROM table LIMIT [offset,] rows 

There are two parameters , First offset , from 0 At the beginning , The second parameter represents the number of rows .

select  * from Student order by Stu_num asc limit 1,2

The result should be the red framed part

The second kind ：SELECT * FROM table LIMIT rows OFFSET offset  

select  * from Student order by Stu_num asc limit 2 offset 1

group by And aggregate functions

 1. In the use of GROUP BY When clause ,Select All columns in the list must be aggregate columns （SUM,MIN/MAX,AVG etc. ） or GROUP BY Columns included in clause . Again , If in SELECT Aggregate columns are used in the list ,SELECT The list must include only aggregate columns , Otherwise there must be one GROUP BY Clause .

2. Aggregate functions . Aggregate functions are used for GROUP BY Words and expressions , Used to aggregate grouped data . The aggregate function is in and GROUP BY Clause shows its powerful function . However, the use of aggregate functions is not limited to grouping queries ; If an aggregate function is used in the query statement , and No use GROUP BY Clause , The aggregate function is used to aggregate the entire result set （ matching WHERE Clause ）. When not used GROUP BY When clause , stay SELECT Some aggregate functions in the list can only be used with other aggregate functions , That is, the aggregate function must use GROUP BY Clause can be used in SELECT Match the list with the list . for example , Don't use GROUP BY Clause ,SELECT In the list AVG And the only SUM Corresponding , But it cannot correspond to a specific column .

Be careful ：  in application , Aggregate functions are often combined with grouping functions group by Use a combination of  

Common aggregate functions

 SQL Introduction to aggregate functions in _ Jianyun's blog -CSDN Blog _ Aggregate functions    What are aggregate functions （aggregate function）？ Aggregate functions perform calculations on a set of values and return a single value .  What are the characteristics of aggregate functions ？ except COUNT outside , The aggregate function ignores null values . Aggregate functions are often associated with SELECT Of the statement GROUP BY Clauses are used together . All aggregate functions are deterministic . Whenever they are called with a given set of input values , All return the same value . Scalar function ： Only a single number or value can be calculated . Lord ...https://blog.csdn.net/qq_40456829/article/details/83657396