MySQL Basic operation -DQL

Concept

• An important function of database management system is data query , Data query should not simply return the data stored in the database , You should also filter the data as needed and determine what format the data is displayed in .
• MySQL Provides powerful functions 、 Flexible statements to implement these operations .
• keyword ：select

Grammar format

--  Basic grammar 
select
	[all|distinct]
	< The expression of the target column 1> [ Alias ],
	< The expression of the target column 2> [ Alias ]...
from < Table or view name > [ Alias ], < Table or view name > [ Alias ]...
[where< Conditional expression >]
[group by < Name >]
[having < Conditional expression >]
[order by < Name > [asc|desc]]
[limit < Number or list >];

--  Concise grammar 
select *| Name  from  Table name  where  Conditional expression 

--  Example 
select * from student; --  Query all fields and columns of the whole table 
select * from student where id = 1; --  accord with ID=1
select id,name from student ; --  Query all the data , Display only id and name Column 

The data sample ：

create database if not EXISTS mydb2;
use mydb2;
create table product(
	pid int(6) PRIMARY KEY auto_increment,
	pname VARCHAR(20) not null,
	price double, 
	category_id varchar(20)
);
INSERT INTO product values(null, ' shirt ', 300, 'c002');
INSERT INTO product values(null, ' Trousers for ', 800, 'c002');
INSERT INTO product values(null, ' Playboy jacket ', 100, 'c002');
INSERT INTO product values(null, ' fleece ', 200, 'c002');
INSERT INTO product values(null, ' Motion pants ', 100, 'c002');
INSERT INTO product values(null, ' Adie shoes ', 900, 'c002');
INSERT INTO product values(null, ' Face cream ', 800, 'c003');
INSERT INTO product values(null, ' Essence water ', 100, 'c003');
INSERT INTO product values(null, ' Chanel ', 200, 'c003');
INSERT INTO product values(null, 'SK-II GHb ', 100, 'c003');
INSERT INTO product values(null, ' Foundation make-up ', 900, 'c003');
INSERT INTO product values(null, ' instant noodles ', 10, 'c004');
INSERT INTO product values(null, ' kelp ', 20, 'c004');
INSERT INTO product values(null, ' The three little squirrels ', 80, 'c004');

Inquire about SELECT

1、 Simple query

--  Query all 
select * from product;
--  Query commodity name and commodity classification pname,category_id
select panme,category_id from product;
--  Column alias 
select pname, price from product;
--  Remove the repetition value  distinct key word 
select distinct price from product; 
--  The query result is an expression ( Arithmetic query ), Put the price of all the goods +10 No operation 
select pname, price+10 from product;

2、 Operator

brief introduction

After the table structure in the database is determined , The meaning of the data in the table has been determined . adopt MySQL Operator to operate , You can get another kind of data besides the table structure .

MySQL Support 4 Operator

• Arithmetic operator ：+ - * / perhaps DIV % perhaps MOD
• Comparison operator
• Logical operators
• An operator

Arithmetic operator

Arithmetic operator	explain
+	Addition operation
-	Subtraction
*	Multiplication
/ perhaps DW	Division operations , Returnee
% perhaps MOD	The remainder , Returns the remainder

Comparison operator

Comparison operator	explain
=	be equal to
< and <=	Less than and less than equal to
> and >=	Greater than and greater than or equal to
<=>	Safe is equal to , Both opcodes are NULL when , The resulting value by 1; And when an opcode is NULL when , The resulting value by 0
<> perhaps !=	It's not equal to
IS NULL perhaps ISNULL	Judge a value Is it NULL
IS NOT NULL	Judge a value If not for NULL
LEAST	When there are two or more parameters , Return minimum
GREATEST	When there are two or more parameters , Return maximum
BETWEEN AND	Judge whether a value falls between two values Between
IN	Judge a value yes IN Any value in the list
NOT IN	Judge a value No IN Any value in the list
LIKE	Wildcard match
REGEXP	Regular Expression Matching

Logical operators

Logical operators	explain
NOT perhaps ！	Logic is not
AND perhaps &&	Logic and
OR perhaps ||	Logic or
XOR	Logical XOR

An operator

Bitwise operators are operators that evaluate on binary , Will first convert the operand to binary , Do bit operations . Then convert the calculation result from binary to decimal . Rarely use .

An operator	explain
!	Press bit or
&	Bitwise AND
^	Bitwise XOR
<<	Move left according to position
>>	Right shift to position
~	According to the not , Invert all bits

3、 Sort query

key word ：order by

• asc Represents ascending order ,desc In descending order , If you don't write, the default is ascending
• order by Used in clauses that can support a single field , Multiple fields , expression , function , Alias
• order by Clause , Put it at the end of the query statement .limit Except clause

Example ：

--  Use price sorting 
select * from product order by price desc;
--  On the basis of price ranking , Sort by category 
select * from product order by price desc, category_id asc;
-- Show the price of the goods （ To repeat ）, And sort 
select distinct price from product order by price desc;

4、 Aggregate query

The queries described above are horizontal queries , They are judged line by line according to the conditions , Using aggregate query is vertical query , It is the value of a column Calculate , And then return a single value ; In addition, aggregate queries ignore null values .

Aggregate functions	effect
count()	The specified column of statistics is not NULL The number of record lines
sum()	Calculates the value of the specified column and , If the specified column type is not a numeric type , So the result is 0
max()	Calculate the maximum value of the specified column , If the specified column is of string type , So use string sort operation
min()	Calculate the minimum value of the specified column , If the specified column is of string type , So use string sort operation
avg()	Calculate the average value of the specified column , If the specified column type is not a numeric type , So the result is 0

Aggregate function pairs NULL value To deal with

• count Function pair null Value handling , If count The argument of the function is asterisk (*), Then count the number of all records . If the parameter is a field , Excluding statistics null value Number of records .
• sum and avg Function pair null value To deal with , These two functions ignore null value The existence of , It's like this record doesn't exist .
• max and min The function also ignores null Existence of value .

5、 Group query group by

Group query refers to the use of group by Group subqueries with words .

Format ：

select  Field 1,  Field 2 ... from  Table name  group by  Grouping field  having  Grouping conditions ;

Example ：

--  Count the number of commodities in each category 
select category_id, count(*) from product group by category_id;

After grouping , be select After Clause , Only grouped fields and aggregate statistical functions can appear , Other fields cannot appear .

Condition filtering after grouping having

• If the statistical results are filtered after grouping, you must use having, Out of commission where
• where Clause is used to filter FROM The row produced by the operation specified in clause
• group by Clause is used to group WHERE The output of the clause
• having Clause is used to filter rows from the grouping results

Format ：

select Field 1, Field 2... from Table name group by Grouping field having Grouping conditions ;

operation ：

--  Count the number of commodities in each category , And only show the number greater than 1 Information about 
select category_id, count(*) from product group by category_id having count(*) > 1;
--  Count the number of commodity categories , Total number of articles , Total sum , And add having Filter multiple criteria , The sum of total classified amounts is greater than 30
select category_id, count(*), sum(price) from product group by category_id having count(*) >１ and sum(price) > 30;

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6、 Paging query limit

Format

--  The way 1, Before display n strip 
select  Field 1,  Field 2 ... from  Table name  limit n

--  The way 2  Pagination display 
select  Field 1,  Field 2 ... from  Table name  limit m,n
m:  Integers , Indicates which index to start from , Calculation method （ The current page -1) *  Number of bars per page 
n:  Integers , Indicates how many pieces of data to query 

Example ：

--  Before query 5 Bar record 
select * from product limit 5;

--  From 5 Bar start , Show 5 strip 
select * from product limit 5,5;

7、INSERT INTO SELECT sentence

Import data from one table into another table , The target table must exist

Format ：

insert into table2(field1, field2, ...) select value1, value2, ... from table1;

perhaps

insert into table2 select * from table1;

Example ：

--  Create a product2 surface , Will table 1 Insert the product name and price into product2 surface 

CREATE TABLE PRODUCT2(
	pname varchar(20),
	price double
);

insert into product2 (pname, price) select pname, price from product;

Example 2：

--  Create a product3 surface , Statistics of commodity classification and quantity 
CREATE table product3(
	category_id varchar(20),
	product_count int
);

insert into product3 select category_id, count(*) from product group by category_id;