MySQL learning record (III) multi table query, sub query, paging query, case statement, single line function

Tips ： Several statements were entered , But I don't want to execute , It can't be deleted , You need to use \c

mysql> SELECT
    ->
    -> \c
mysql>

Multi-table query

Write the connection conditions in where Later, this method is called implicit connection , Its function is the same as that of internal connection , The belt on The connection mode of is called explicit connection . Now with the standardization and development of database , Most use explicit connection , Rarely use implicit connections .

1. Cartesian product

When multi table join query , If an invalid connection or missing connection condition is defined , Will produce Cartesian product phenomenon . The so-called Cartesian product is that each row of each table is combined with each row of other tables , Suppose the total number of rows in two tables is one x, One y, Returns the Cartesian product x*y.

2. Equivalent join query

Find the field names in the two tables are equal , Connect fields with the same data type , Will automatically go to multiple columns （ If more than one field meets the requirements , Then they will be regarded as the condition of natural connection ） Also called natural connection

The most common , It is usually carried out between tables with primary key and foreign key associations , And set the connection condition to the related column 【 Primary key - Foreign keys 】, Use ”=“ Connect related tables . Avoid Cartesian product ,n At least n-1 Equivalent connection conditions .

mysql>-- emp Employee table  dept_id  The fields are  dept The primary key of the Department table  id
mysql> desc dept;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| DEPTNO   | int         | NO   | PRI | NULL    |       |
| DNAME    | varchar(14) | YES  |     | NULL    |       |
| LOCATION | varchar(13) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> desc emp;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| EMPNO    | int         | NO   | PRI | NULL    |       |
| ENAME    | varchar(10) | YES  |     | NULL    |       |
| JOB      | varchar(9)  | YES  |     | NULL    |       |
| MANAGER  | int         | YES  |     | NULL    |       |
| HIREDATE | date        | YES  |     | NULL    |       |
| SAL      | double(7,2) | YES  |     | NULL    |       |
| COMM     | double(7,2) | YES  |     | NULL    |       |
| DEPTNO   | int         | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
8 rows in set (0.00 sec)

mysql> --  The writing method of implicit connection 
mysql> select d.dname, e.empno, e.ename from dept as d, emp e where d.deptno = e.deptno;
+------------+-------+--------+
| dname      | empno | ename  |
+------------+-------+--------+
| RESEARCH   |  7369 | SMITH  |
| SALES      |  7499 | ALLEN  |
| SALES      |  7521 | WARD   |
| RESEARCH   |  7566 | JONES  |
| SALES      |  7654 | MARTIN |
| SALES      |  7698 | BLAKE  |
| ACCOUNTING |  7782 | CLARK  |
| RESEARCH   |  7788 | SCOTT  |
| ACCOUNTING |  7839 | KING   |
| SALES      |  7844 | TURNER |
| RESEARCH   |  7876 | ADAMS  |
| SALES      |  7900 | JAMES  |
| RESEARCH   |  7902 | FORD   |
| ACCOUNTING |  7934 | MILLER |
+------------+-------+--------+
14 rows in set (0.00 sec)

as Give an alias , Generally, the aliases of tables are directly separated by spaces Omit as

3. Self connect query

Query multiple tables in one table ,.

Employees and their superiors are in the employee table , use manager_id To identify employees' superiors

select e1.empno as  Employee number , e1.ename as  Employee name , e2.empno as  Led number , e2.ename as  Leader's name  from emp e1, emp e2 where e1.manager_id = e2.empno;

4. Internal connection query

keyword ： surface 1 inner join surface 2 on Connection condition (inner It can be omitted )

sentence ：select * from a_table a inner join b_table b on a.a_id = b.b_id;

explain ： Combine records from two tables , Return the records matching the associated fields , That is, return the intersection of two tables .
Self join query can also be realized by inner join ：

SELECT e1.empno AS  Employee number , e1.ename AS  Employee name , e2.empno AS  Led number , e2.ename AS  Leader's name  FROM emp e1 JOIN emp e2 ON e1.manager_id = e2.empno;

The condition is not necessarily that the two tables are primary and foreign key relationships , The records that meet the conditions are returned

5. External connection query

The external connection is divided into left outer join（ The left outer join ） and right outer join （ Right connection ）,outer It can be omitted .

keyword join The one on the left is called the left watch , The one on the right is called the right watch

a. The left connection indicates that all data of the left table is displayed , The data meeting the connection conditions in the right table , Dissatisfied use null Value padding ; Right connection indicates that all data in the right table is displayed , The data that meets the connection conditions in the left table , dissatisfaction null Value padding .

b. The result set should contain all matching rows in the right table , If some items in the right table have no corresponding items in the left table , With null Value padding .
ad locum ,join The specified table is the main table , That is, the right table is the main table .（ Which table shows all rows , It's the main table ）

mysql> --  The left outer join  ( OUTER  Keywords can be omitted  )
mysql> SELECT * FROM t_class c LEFT OUTER JOIN t_major m ON c.major_id = m.id ;
+----+----------+----------+------+------------------+
| id | name     | major_id | id   | name             |
+----+----------+----------+------+------------------+
|  1 |  Software class one  |        2 |    2 |  Software Engineering          |
|  2 |  Software class two  |        2 |    2 |  Software Engineering          |
|  3 |  Class 1 of planning department  |        4 |    4 |  Computer science and technology  |
|  4 |  Class 2 of planning department  |        4 |    4 |  Computer science and technology  |
|  5 |  Excellence class    |     NULL | NULL | NULL             |
|  6 |  Dongpo class    |     NULL | NULL | NULL             |
+----+----------+----------+------+------------------+
6 rows in set (0.00 sec)

mysql> --  Right connection  ( OUTER  Keywords can be omitted  )
mysql> SELECT * FROM t_class c RIGHT OUTER JOIN t_major m ON c.major_id = m.id ;
+------+----------+----------+----+------------------+
| id   | name     | major_id | id | name             |
+------+----------+----------+----+------------------+
| NULL | NULL     |     NULL |  5 |  Information confrontation          |
| NULL | NULL     |     NULL |  3 |  Animal medicine          |
| NULL | NULL     |     NULL |  1 |  Civil Engineering          |
|    3 |  Class 1 of planning department  |        4 |  4 |  Computer science and technology  |
|    4 |  Class 2 of planning department  |        4 |  4 |  Computer science and technology  |
|    1 |  Software class one  |        2 |  2 |  Software Engineering          |
|    2 |  Software class two  |        2 |  2 |  Software Engineering          |
+------+----------+----------+----+------------------+
7 rows in set (0.00 sec)

Full outer join ( MySQL I won't support it )
It is equivalent to the union of left outer and right outer connections
SELECT * FROM t_class c FULL OUTER JOIN t_major m ON c.major_id = m.id ;

Subquery

1. Single line sub query

Return single row and single column

2. Multi line sub query

Return single row and multiple columns ( Paired comparisons are used ))

3. Multi column subquery

Return multiple rows and single column

4. Correlation subquery

Subqueries use external SQL Some tables or columns of ( But outside SQL Tables and columns in subqueries cannot be used )

5. nested subqueries :

You can continue to embed subqueries inside subqueries

Paging query

Use lilmit Realize paging query
grammar ：limit a , b The first parameter indicates which item to start from （ Index with pages （0 Start ）* Records per page ）, The second one means to check several items on one page

--  Display the first to fifth records 
select * from emp limit 0,5;

CASE sentence

1. grammar ：

CASE 
	WHEN  Conditions 1 THEN  result 1
	WHEN  Conditions 2 THEN  result 2
	...
	WHEN  Conditions n THEN  result n
	ELSE  Other cases 
END	

2. Example ：

mysql> SELECT s.name  The student's name  ,
    ->        c.name  Course name  ,
    ->        CASE
    ->            WHEN x.score BETWEEN 90 AND 100 THEN 'A'
    ->            WHEN x.score > 80 THEN 'B'
    ->            WHEN x.score > 70 THEN 'C'
    ->            WHEN x.score > 60 THEN 'D'
    ->            ELSE 'E'
    ->        END AS  Grades 
    -> FROM t_students s
    ->      JOIN t_scores x ON x.sid = s.id
    ->      JOIN t_courses c ON x.cid = c.id ;
+--------------+--------------+--------------+
|  The student's name      |  Course name      |  Grades      |
+--------------+--------------+--------------+
|  zhang wuji        | C++          | A            |
|  zhang wuji        | C++          | A            |
|  Zhou Zhiruo        | C++          | A            |
|  Zhang Sanfeng        | C++          | A            |
|  Liu Shumei        | C++          | E            |
|  Liu Shumei        | C++          | E            |
|  zhang wuji        | Java         | A            |
|  Small zhao          | Java         | A            |
|  Wu Mochou        | Java         | D            |
|  Zhao Min          | Oracle       | B            |
|  Zhang Sanfeng        | Oracle       | E            |
|  Zhou Zhiruo        | HTML/CSS     | A            |
|  Wu Mochou        | HTML/CSS     | C            |
|  Liu Shumei        | HTML/CSS     | E            |
+--------------+--------------+--------------+
14 rows in set (0.00 sec)

One line function

SQL Function is the built-in function of the database , It can be applied to SQL Statement to achieve specific functions , The single line function calculates the data of each line and gets a line of output ; A multi line function is a function in which multiple lines of data participate in the operation to obtain a line of output .

1. Character functions

length()、 concat()、upper() 、lower()、substr() or substring()、 instr()、trim()、lpad() and rpad()、replace() ;

Case conversion function ：upper,lower

mysql> SELECT upper('abc'), lower('ABC');
+--------------+--------------+
| upper('abc') | lower('ABC') |
+--------------+--------------+
| ABC          | abc          |
+--------------+--------------+
1 row in set (0.00 sec)

String concatenation function ：concat

Connection string

mysql> SELECT concat('tx', 'sql');
+---------------------+
| concat('tx', 'sql') |
+---------------------+
| txsql               |
+---------------------+
1 row in set (0.00 sec)

Get substring function ：substr

Used for string interception The first parameter is the truncated string The second parameter is the number of bits to intercept Space is also a character

mysql> SELECT substr('java sql js', 5);
+--------------------------+
| substr('java sql js', 5) |
+--------------------------+
|  sql js                  |
+--------------------------+
1 row in set (0.00 sec)

Other string functions ：

1. length( character string ); ： Gets the length of the string
2. instr(' The parent string ', ' Substring '); ： Get the index of the substring in the parent string from 1 Start The space is also called
3. ltrim(' java web'); ： Remove the space to the left of the string
4. rtrim('java web '); ： Remove the space on the right
5. trim(' java web '); ： Remove the spaces on both sides
6. replace(' character string ', ' Substring to replace ', ' Replace string '); ： The first is the parent string The second substring to replace Replaced string

2. Mathematical functions

round()、ceil() and floor()、truncate()、mod();

• round(x, [y]) ： For the specified X Perform rounding operations You can specify the number of reserved digits y It can be negative
• truncate(x, y) ： For the specified X Intercept operation , You can specify the number of digits to be reserved
• ceil(x) ： Return no less than the specified value x Minimum integer of such as x yes 4.5 return 5
• floor(x) ： Return is no greater than x Maximum integer for such as x yes 8.1 return 8
• abs(x) ： Take the absolute value
• mod(x, y) ： take x Divide y The remainder of
• sign(x) ： take x Symbolic function of negative （-1）,0, Positive numbers （1）
• power(x, y) ： take x Of y The next power x=2 ,y=3, be 2 Of 3 The second power equals 8
• sqrt(x) ： take x The square root of

3. Date function

now()、curdate()、curtime()、 year 、 month 、 Japan 、 Hours 、 minute 、 second 、str_to_date() and date_format();

The current date function

Date interception function

Date increasing function

special case ：

Date formatter

1. Concept ：

Formatter is a format symbol that describes the date , Use letters to describe specific parts of the date , With % start , for example %m Represents the month in the date .

2. Formatter table ：

Format	describe
%a	Abbreviated week name
%b	Abbreviated month name
%c	month , The number
%D	Days of the month with English prefix
%d	Day of the month , The number （00-31）
%e	Day of the month , The number （0-31）
%f	Microsecond
%H	Hours （00-23）
%h	Hours （01-12）
%I	Hours （01-12）
%i	minute , The number （00-59）
%j	Days of （001-366）
%k	Hours （0-23）
%l	Hours （1-12）
%M	Month name
%m	month , The number （00-12）
%p	AM or PM
%r	Time ,12- Hours （hh:mm:ss AM or PM）
%S	second （00-59）
%s	second （00-59）
%T	Time , 24- Hours （hh:mm:ss）
%U	Zhou （00-53） Sunday is the first day of the week
%u	Zhou （00-53） Monday is the first day of the week
%V	Zhou （01-53） Sunday is the first day of the week , And %X Use
%v	Zhou （01-53） Monday is the first day of the week , And %x Use
%W	Week name
%w	Days of the week （0= Sunday , 6= Saturday ）
%X	year , Sunday is the first day of the week ,4 position , And %V Use
%x	year , Monday is the first day of the week ,4 position , And %v Use
%Y	year ,4 position
%y	year ,2 position

Query today

/*
  NOW()  Function returns the current date and time .
  TO_DAYS() Function returns the date and year 0（ date "0000-00-00"） Days between .
*/
SELECT * FROM cpidata WHERE TO_DAYS(  Time field name  ) = TO_DAYS(NOW());

Check yesterday

/*
  NOW()  Function returns the current date and time .
  TO_DAYS() Function returns the date and year 0（ date "0000-00-00"） Days between .
*/
SELECT * FROM  Table name  WHERE TO_DAYS( NOW( ) ) - TO_DAYS(  Time field name ) = 1

Check this week

/*
  YEARWEEK() Return year and week 
  DATE_FORMAT( Formatted date , Format symbol ) Date format function 
*/
SELECT * FROM  Table name  WHERE YEARWEEK(DATE_FORMAT( Date field name ,'%Y-%m-%d')) = YEARWEEK(NOW());

Query last week

/*
  YEARWEEK() Return year and week 
  DATE_FORMAT( Formatted date , Format symbol ) Date format function 
*/
SELECT * FROM  Table name  WHERE YEARWEEK(DATE_FORMAT( Date field name ,'%Y-%m-%d')) = YEARWEEK(NOW())-1;

Query near 7 God

/*
  DATE_SUB()  Function to subtract a specified time interval from a date 
  DATE_SUB( Legal date expression ,INTERVAL  The interval you want to add   Time type )
  CURDATE()  Function returns the current date .
*/
SELECT * FROM  Table name  where DATE_SUB(CURDATE(), INTERVAL 7 DAY) <= date( Time field name )

Query near 30 God

/*
  DATE_SUB()  Function to subtract a specified time interval from a date 
  DATE_SUB( Legal date expression ,INTERVAL  The interval you want to add   Time type )
  CURDATE()  Function returns the current date .
*/
SELECT * FROM  Table name  where DATE_SUB(CURDATE(), INTERVAL 30 DAY) <= date( Time field name )

Inquire about this month

/*
  DATE_FORMAT( Formatted date , Format symbol ) Date format function 
*/
SELECT * FROM  Table name  WHERE DATE_FORMAT(  Time field name , '%Y%m' ) = DATE_FORMAT( CURDATE( ) , '%Y%m' );

Query last month

/*
  PERIOD_DIFF( First date , Second date ) Function returns the difference between two dates . The results are calculated in months 
*/
SELECT * FROM  Table name  WHERE PERIOD_DIFF( DATE_FORMAT( NOW( ) , '%Y%m' ) , DATE_FORMAT(  Time field name , '%Y%m' ) ) =1

Query this quarter

/*
 QUARTER( date ) Returns the quarter of the date ：
*/
SELECT * FROM  Table name  WHERE QUARTER( Date field name )=QUARTER(NOW());

Query last quarter

/*
 QUARTER( date ) Returns the quarter of the date ：
 DATE_SUB()  Function to subtract a specified time interval from a date 
 DATE_SUB( Legal date expression ,INTERVAL  The interval you want to add   Time type )
*/
SELECT * FROM  Table name  WHERE QUARTER( Date field name )=QUARTER(DATE_SUB(NOW(),INTERVAL 1 QUARTER));

Query current year

/*
 year( Date field name ) Returns the year in the date 
*/
SELECT * FROM  Table name  WHERE YEAR( Date field name )=YEAR(NOW());

Query last year

/*
 year( Date field name ) Returns the year in the date 
 DATE_SUB()  Function to subtract a specified time interval from a date 
 DATE_SUB( Legal date expression ,INTERVAL  The interval you want to add   Time type )
*/
SELECT * FROM  Table name  WHERE YEAR( Date field name )=YEAR(DATE_SUB(NOW(),INTERVAL 1 YEAR));

4. Other built-in system functions

version()、database()、user();