I study database at station B (4): DQL

Last one ： I am here b Station learning database （ 3、 ... and ）： constraint

Data preparation

1、 Create databases and tables

2、 Add data

Two 、 Simple query

1、 Check all products （ Generally, the method of choosing the second line ）

result ：
2、 Query the product name and price

result ：
3、 Alias query , Use keywords as,as Omission
（1） Table alias

（2） List aliases

4、 Remove the repetition value （-distinct）


5、 Arithmetic query

3、 ... and 、 Operator operation

1、 Arithmetic operator

example ： Add... To the price of all goods 10 element


2、 An operator （ Understanding can ）

3、 For comparison operators and logical operators, see the following conditional query

Four 、 Conditions of the query

1、 Query the trade name as “ Haier washing machine ” All information about our products

2、 Inquiry price is not 800 All of our products （ Three lines are OK , Usually choose the first line ）


3、 The inquiry price is greater than or equal to 60 The goods

4、 Inquire about the price of goods in 200 To 1000 All goods between

5、 The inquiry of commodity price is 200 or 800 All of our products

6、 Query contains ‘ pants ’ All the goods of the word

7、 Query to ‘ The sea ’ All products beginning with the word

8、 The second word is ‘ Kou ’ All of our products

9、 Use least For the minimum

10、 Use greatest For maximum

5、 ... and 、 Sort query

1、 Single condition

2、 Multiple conditions

3、 To reorder

6、 ... and 、 Aggregate query

1、 Concept
Aggregate function queries are vertical queries , It calculates the values of a column , And then return a single value . Besides , Aggregate functions ignore null values .
2、 Aggregate functions

Be careful ：（1）count The argument of the function is asterisk （*）, Then count the number of all records . If the parameter is a field , Excluding statistics null Number of records of value .
（2）sum、max、min and avg Function to ignore null Existence of value .

3、 example

7、 ... and 、 Group query

1、 Count the number of goods in each category


Be careful ： After grouping select Only grouping fields and aggregate functions can be written behind the

2、 Count the number of commodities in each category , And only show that the number is greater than 4 Information about

8、 ... and 、 Paging query

1、 Format
Before display n strip ：
select Field 1, Field 2… from indicate limit n
Pagination display ：
select Field 1, Field 2… from indicate limit m,n

m: Integers , Indicates which index to start from , Calculation method （ The current page -1）
n: Integers , Indicates how many pieces of data to query

2、 operation
（1） Inquire about product The first five records of the table

（2） Start with the fourth line , Show 5 strip

Nine 、insert into select sentence

1、 brief introduction
When importing data from one table into another table
2、 Format

Be careful ： Request target table Table2 There must be
3、 example

Ten 、 summary