Single-table queries
Simple Query
select * from table; See table all the data
select field from table; single view field data
select field, the field, from the field name; a plurality of view field data
select distinct from the field name; check out of the fields to re- select name, money * 12 from table; field may be four arithmetic select concat (field, 'string', fields) from table; field name field string data as output fields long, can from the field name before adding a new or as new field name select concat_ws ( 'separator', 'string', fields, field) from simple field names name; the data string delimiter field separator field output in the form case is similar if select (case when conditions then results When name = 'Messi' then the concat (name, '_ The Best Player' ) the else the concat (name, '_great Player' ) End) AS NEW_NAME from table; if the field name is Messi, Messi is the output of the world's best players otherwise the output name is a good player
where constraints
select * from table where conditions
Range query
> <> = <= =! = / <>
BETWEEN A and B
in (1,2,3,4 ) is selected from n-1 Fuzzy queries like % arbitrary character representative of a percent sign of any length 'A%' ' ING% ' '% a% ' _ represents an optionally an underscore character' A__ ' regexp' ^ a '' \ + D 'IS IS IS Not null IS Not null logical operations and or not
Group by group
Carried out in accordance with a high repetition rate field
How many in this field might be divided into how many groups
Deduplication can be done
Once the packet can not be on a specific piece of data to operate, and will always consider this group xxx
If we use the field as a unique basis for grouping, each a self-contained set of records, this grouping does not make sense
the same value of a field between multiple records, the field is usually used as a basis for grouping
group_concat: only used for the final display, the other data can not be operated as an intermediate result
Alone GROUP BY keyword grouping
SELECT post FROM employee GROUP BY post;
Note: we follow the post field grouping, the field can only post select query, you want to get additional relevant information within a group, need the help function
GROUP BY keyword and the GROUP_CONCAT () using a function with
SELECT post, GROUP_CONCAT (emp_name) FROM employee gROUP BY post; # accordance job packet, and view group members name of the SELECT POST, the GROUP_CONCAT (the emp_name) AS emp_members the FROM Employee the gROUP BY POST;
the gROUP BY polymerization use function with
select post, count (id) as count from employee group by post; # grouped according to the post, and see how many people in each group
polymerization
99.99% of cases are used, and grouped together
if not together with the packet and, by default the entire table is a set of a
COUNT (ID) / COUNT (* ) Count: each group of data corresponding to several
max selecting the maximum value: the the maximum value of a field of the group
min selecting the maximum value: in this group the minimum value of a field
avg averaged
sum summation value
Filter conditions having
Condition is a screening of the groups
WHERE HAVING not the same place and that the
execution priority from high to low: WHERE> Group by> HAVING
. 1 . Where occurs before the packet group by, and thus there can Where any field, but must not use the aggregate functions.
2. Having occurs after a packet group by, Having thus can be used in the field of the packet, can not be taken directly to other fields, aggregate functions can be used
Sort order by inquiry
Field order by sort fields, default ascending
order by field for the field in ascending order asc
order by field desc descending order of the field
order by one field, two fields 2 fields ascending
order by field asc, desc Field 2 Field ascending are arranged in descending order of field 2
Limit the query to limit the number of records
1 . Display tab limit m, n represents the m + 1 starts, taking the n limit 0, 6 represents six taken from a start limit 6,6 represents from 7 to begin fetching six limit 12,6 represents from 13 to begin fetching 6 Article limit 18,6 represents from 19 starts to take 6 2 from the former name n. the default is 0 limit nm with the order by, together with the limit n offset m: m from start + 1, taking the n
Multi-table query
Construction Table Create Table Department ( ID int, name VARCHAR ( 20 is ) ); Create Table Employee ( ID int Primary Key AUTO_INCREMENT, name VARCHAR ( 20 is ), Sex enum ( ' MALE ' , ' FEMALE ' ) Not null default ' MALE ' , int Age, the dep_id int ); insert data iNSERT INTO Department values ( 200 is, ' technical ' ), ( 201, ' HR ' ), (202,'销售'), (203,'运营'); insert into employee(name,sex,age,dep_id) values ('egon','male',18,200), ('alex','female',48,201), ('wupeiqi','male',38,201), ('yuanhao','female',28,202), ( ' Liwenzhou ' , ' MALE ' , 18,200 ), ( ' jingliyang ' , ' FEMALE ' , 18,204 ) ;
Even table query
The two tables together, prior calculated Cartesian product of two tables, and then filtered according to the conditions given by the user
Turned out to be two tables, and now makes up a table, all operations are the same as the operation of a table
A cross-connect: NA any matching condition. Cartesian product is generated
MySQL> SELECT * from Employee, Department; + ---- + ----------------------- + -------- + + ------ ------ - + ------ + -------------- + | the above mentioned id | name | Sex | Age | dep_id | the above mentioned id | name | + ---- + ---- -------- + -------- + -------- + ------ + ------ + --------- + ----- |. 1 | Egon | MALE | 18 is | 200 is | 200 is | art | |. 1 | Egon | MALE | 18 is | 200 is | 201 | HR | |. 1 | Egon | MALE | 18 is | 200 is | 202 | sales | | 1 | Egon | MALE | 18 | 200 | 203 | operations | | 2 | alex | FEMALE | 48 | 201 | 200 | technology | | 2 | alex | FEMALE | 48 | 201 | 201 | human resources | | 2 | alex | female | 48 | 201 | 202 | sales | | 2 | alex | female | 48 | 201 | 203 | operations | |. 3 | wupeiqi | MALE | 38 is | 201 | 200 is | art | |. 3 | wupeiqi | MALE | 38 is | 201 | 201 | HR | |. 3 | wupeiqi | male | 38 | 201 | 202 | sales | |. 3 | wupeiqi | MALE | 38 is | 201 | 203 | operator | |. 4 | yuanhao | FEMALE | 28 | 202 | 200 is | art | |. 4 | yuanhao | FEMALE | 28 | 202 | 201 | HR | |. 4 | yuanhao | FEMALE | 28 | 202 | 202 | sales | |. 4 | yuanhao | FEMALE | 28 | 202 | 203 | operator | |. 5 | liwenzhou | MALE | 18 is | 200 is | 200 is | technology | | 5 | liwenzhou | MALE | 18 | 200 | 201 | HR | | 5 | liwenzhou | male | 18 | 200 | 202 | Sales | | 5 | liwenzhou | MALE | 18 | 200 | 203 | operations | | 6 | jingliyang | female | 18 | 204 | 200 | Technology | | 6 | jingliyang | FEMALE | 18 | 204 | 201 | Human Resources | | 6 | jingliyang | FEMALE | 18 | 204 | 202 | Sales | | 6 | jingliyang | female | 18 | 204 | 203 | operations | + ---- + -------- + ------------ + ------ + ---- ---- + ------ + -------------- +
2 the connection: only matching line connected
There are two tables to find parts, the equivalent of using the criteria to filter out results from the Cartesian product of the correct results department this department is not 204, so the employee table about 204 employees this information does not match up
show only two tables match each other item not displayed does not match the other MySQL> SELECT employee.id, employee.name, employee.age, employee.sex in, department.name from the Join Department Employee Inner ON employee.dep_id = department.id; + ---- + + -------- + ------ + ----------- + -------------- | the above mentioned id | name | Age | Sex | name | + ----------- + ------ + ---- + -------- + ------------ - + |. 1 | Egon | 18 is | MALE | art | | 2 | Alex | 48 | FEMALE | HR | |. 3 | wupeiqi | 38 is | MALE | HR | |. 4 | yuanhao | 28 | FEMALE | sales | | 5 | liwenzhou | 18 | male | technology | + ---- ------ + + -------- + + ----------- + -------------- above sql Equivalent to mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;
3 external link Zhizuo connection: priority display all records from the left table
To the left table subject, that is, to find all the employee information, including of course no sector employees is essentially: increase the basal including the right connection on the left there is no result
regardless of whether the match left on the table will display all content mysql> select employee.id, employee.name, department.name AS depart_name left from the Join Employee Department employee.dep_id ON = department.id; + ---- + ---- + ----------------------- + ---------- | the above mentioned id | name | depart_name | + ---- + ------------ + ------------- - + | 1 | Egon | technology | | 5 | liwenzhou | technology | | 2 | alex | human resources | | 3 | wupeiqi | human resources | | 4 | yuanhao | sales | | 6 | jingliyang | NULL | + --- - + ------------ + -------------- +
4 connected to the right hand outer link: all records displayed by priority right table
Subject to the right table, that is, to find information in all sectors, including the lack of staff departments essence is this: there is left no increase to the right of the results on the basis of connections, including
on whether or not to match the right table will display all content mysql> select employee .id, employee.name, department.name AS depart_name right from the Join Employee Department employee.dep_id ON = department.id; + ------ + ----------- + ---- + ---------- | the above mentioned id | name | depart_name | + ------ + ----------- + ------------ - + | 1 | Egon | technology | | 2 | alex | human resources | | 3 | wupeiqi | human resources | | 4 | yuanhao | sales | | 5 | liwenzhou | technology | | NULL | NULL | operations | + - ---- + ----------- + -------------- +
5 full outer join: All records about two tables
Full outer join: including increasing the basic connection on the left has the right results to the right and not the left there is no Note: mysql does not support full outer join full JOIN stressed: mysql this way can be used indirectly to achieve full outer joins select * from employee ON = employee.dep_id the Join Department left department.id Union SELECT * from the Join Department Employee right employee.dep_id ON = department.id ; see results + ------ + ------------ -------- + -------- + ------ + + ------ + -------------- + | the above mentioned id | name | Sex | Age | dep_id | the above mentioned id | name | + ------ + ------------ + -------- + ------ + + ------ + -------------- + -------- |. 1 | Egon | MALE | 18 is | 200 is | 200 is | art | |. 5 | liwenzhou | male | 18 | 200 | 200 | technology | | 2 | alex | FEMALE | 48 | 201 | 201 | HR | | 3 | wupeiqi | male | 38 | 201 | 201 | HR | | 4 | yuanhao | female | 28 | 202 | 202 | Sales | |. 6 | jingliyang | FEMALE | 18 is | 204 | NULL | NULL | | NULL | NULL | NULL | NULL | NULL | 203 | operator | + ---- - + ------------ + -------- + -------- + ------ + ------ + - ------------ + Note the difference between union and the union all: union will remove the same record
Eligible join query
Example 1: connected within query tables employee and department, and employee age field value in the table must be greater than 25, that is, to find older than 25 years of department employees and staff where
select employee.name, department.name from employee inner department the Join
ON employee.dep_id = department.id
WHERE age> 25;
example 2: query connected within employee and department tables and displayed in ascending order of the age field
select employee.id, employee.name, employee.age, department Employee from .name, Department
WHERE employee.dep_id = department.id
and Age> 25
Order by Age ASC;
Subqueries
Sub-query is a query nested within another query statement
The inner query results statement, the query may provide the conditions for the outer query
Subqueries can include in, not in, exists and comparison operators and other keywords
1 IN keyword with a subquery
Queries average age of 25-year-old department name
the SELECT the above mentioned id, name from Department
the WHERE the above mentioned id in
(the SELECT dep_id from the Employee Group by dep_id the HAVING AVG (Age)> 25);
see technical department employee name
the SELECT name from the Employee
the WHERE dep_id in
(the SELECT id from department where name = 'technology');
see less than one person's department name (subquery get is someone's department the above mentioned id)
the SELECT name from the above mentioned id not in the WHERE department (dep_id the SELECT DISTINCT from the Employee);
2 subqueries with comparison operators
Comparison operators:! =, =,>,> =, <, <=, <> Query is greater than the average age for all employees name and age mysql> select name, age from emp where age> (select avg (age) from EMP); + --------- + ------ + | name | Age | + --------- + ------ + | Alex | 48 | | wupeiqi | 38 | + --------- + ------ + 2 rows in the SET (0.00 sec) query is greater than the average age of employees within the department name, age select t1.name, t1.age from T1 EMP Inner the Join (SELECT the dep_id, AVG (Age) avg_age from Group by the dep_id EMP) T2 ON = t2.dep_id t1.dep_id WHERE t1.age> t2.avg_age;
3 with keyword EXISTS subquery
Off EXISTS word indicates the presence of the cross key. When using the EXISTS keyword, the inner query does not return records query.
But return a true or false value. True or False
when returns True, the outer query query; when the return value is False, the outer query is not a query
department表中存在dept_id=203，Ture
mysql> select * from employee
-> where exists
-> (select id from department where id=200);
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+
department表中存在dept_id=205，False
mysql> select * from employee
-> where exists
-> (select id from department where id=204);
Empty set (0.00 sec)