MySQL quicklearn-2021-09-01

0. Abstract

a. Sort

b. Sort + Screening

c. Single line processing functions （ Data processing functions ）

d. Multiline processing functions （ Union function ） And group query

e.distinct keyword

1. Sort , Sorting and screening

order by fieldName asc;	according to fieldName The fields are arranged in ascending order . Sorting and of strings C,Java similar .
order by fieldName desc;	according to fieldName The fields are arranged in descending order
order by fieldName;	according to fieldName The fields are arranged in ascending order （ The default is ascending ）
order by field1 asc, field2 asc...;	First of all, in accordance with the field1 Ascending order of （ It can also be in descending order ） Arrange ; In case of equality , Again according to field2 Ascending order of （ It can also be in descending order ） Arrange ... In short, the priority written in front is high .
select field1, field2,... from tableName order by ...	Arrange the table in the specified order of the specified fields

example ： surface stuinfor as follows

+------+-----+--------+
| name | age | sex    |
+------+-----+--------+
| Guo  |  19 | female |
| Wen  |  25 | female |
| Xu   |  21 | male   |
| Ying |  21 | female |
+------+-----+--------+

First according to age Sort , Again according to name Conduct .

SELECT * FROM stuinfor ORDER BY age desc, name\g

                         Screening + Sort

SELECT filed1,field2,... FROM tableName WHERE expr ORDER BY...

Filter and sort , The order of keywords cannot be changed .

The execution order of the above statements is ：FROM( Find table )--WHERE( Filter by criteria )--SELECT( lookup )--ORDER BY( Sort ). The final output . Note that the order of keywords cannot be changed .

For example, rank the women in the above table ：

SELECT * FROM stuinfor WHERE sex = 'female' ORDER BY age desc\g

2. Single line processing functions

Single line processing functions , Also known as data processing function , Different from multi line processing function . Its characteristics are ： An input , An output ; Several inputs , Several outputs . Although the teacher didn't emphasize , But a single line handler should have formal parameters , Arguments and return values .

The function prototype	function	other
String lower(String);	Convert a string from uppercase to lowercase	Encountered non letters remain unchanged
String upper(String);	Convert a string from lowercase to uppercase	Encountered non letters remain unchanged
String substr(String,left,length);	Take the substring of the given character , And back to	about mysql, The subscript of the string is from 1 Start instead of 0.
int length(String);	Returns the length of the specified string	The difference in java Medium object.length.
String trim(String);	Remove spaces from the given string , And back to	It is often used for preliminary processing of input data . Remove the space before and after . The middle space cannot be removed .
double rand(); double rand(int);	Generate [0,1)	Generate non repeatable random numbers in the default mode ,rand(const_n) Generate repeatable random numbers .
double round(double,int);	Round to the specified number of digits	Rounds the given floating-point number to the specified number of digits ,0 For a bit ,1 Represents ten ,2 Represents the percentile ......-1 On behalf of ten ,-2 Represent hundreds .
double ifnull(data,double)	Check whether the specified data is bit NULL. if NULL Returns the specified value , Otherwise return the original value .	because mysql in NULL Participate in arithmetic operations , The result must be NULL. To avoid this phenomenon , Therefore adopt ifnull function .
case...when...then... when...then...else...end	take case The following variables are related to when After the match , And the corresponding then The value of the expression after is taken as the value of the whole function ; otherwise , take else The value after is taken as the value of the whole function . As long as a when If the condition is met, the following statement will be skipped , Direct to end.	about else Is it a necessary question ： Not required . when Followed by the value , Instead of logical expressions ;then It should be followed by value ;case Followed by variables or fields . namely ： case fieldName when fieldValue then expr1 else expr2;

Single line handlers can form expressions , Can be used wherever expressions can be used . such as select after ,where Then, together with logical operators, they form logical expressions, and so on .

3. Multiline processing functions

The last part of single table query . Multi line processing functions are different from single line processing functions , Multiple input, One output. There are a total of 5 individual , as follows ：

int count(fieldName);	Return the specified field that is not NULL The number of elements
double sum(fieldName);	Return the specified field that is not NULL The sum of the elements of
double avg(fieldName);	Returns the average value of the elements in the specified field , Don't consider NULL Elements
double max(fieldName);	Returns the value of the largest element in the specified field
double min(fieldName);	Returns the value of the smallest element in the specified field

Several considerations for using grouping functions ：

        1. When using the grouping function, it does not consider that the field is NULL The elements of , So you don't need to use it ifnull Pre treatment ;

        2. Grouping function must be grouped before use . Grouping needs group by keyword , If it is not used, the whole table is a group by default .

        3.where The grouping function cannot be used later , This is related to the execution order of various statements . It will be elaborated later .

There are two new keywords in the group query ：

group by fieldName1, fieldName2, ......;	Group according to the specified field . The number of groups should be the product of the number of groups of each field . As before select, You can only take group by Fields after .
having	Grammatically similar to where, For conditional screening , The difference lies in the execution order of statements .

4. Group query

Consider the execution order of the following statements ：

        select ......

        from ......

        where ......

        group by ......

        having ......

        order by ......\g

The order of arrangement cannot be changed , The order of execution is ：1. Use from Find table ;2. Use where To filter （ Filtering immediately after finding the table is conducive to improving the execution efficiency ）;3. Use group by Grouping （ because group by The order of execution is where after , therefore where Grouping functions cannot be used after , Because it violates “ Group first and then use ” Principles ）;4. Use having Further filtering after grouping （ Able to use where Instead, try to use where Instead of , To improve the efficiency of execution ）;5. Use select Take the specified data from the table （ Be careful , With order by after select You can only take group by Fields after ）;6. utilize order by Sort . The final output .

There's a question to think about ：order by Yes, it will sort within the Group , Or overall ranking ？ The whole problem is not true in single table query . Because after grouping, you can only see the statistical information in the Group , Can't see the specific details , Not to mention sorting within groups .

5.distinct keyword

usage ：select distinct fieldName1, fieldName2... from tableName;

distinct Must be placed in all field names （ Variable ） front , Used to remove duplicate records . If there are multiple field names after it , It means that multiple fields jointly remove duplicate records . When multiple fields jointly remove duplicate records , Only all fields of a certain two records （ stay distinct After that ） The values of are all the same before they are considered to be repeated ; As long as there is one field different , Will keep both .

have access to count and distinct Calculate the number of records that are not repeated .

select count(distinct fieldName1,...) from table;