3.1. Simplified supplement to DQL

Catalog

01、 Create table ： Student list (stu)、 Employee list (emp)

02、 Create student table (stu)、 Employee list (emp) Complete data query operation

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01、 Create table ： Student list (stu)、 Employee list (emp)

# Create database myStudent
CREATE DATABASE IF NOT EXISTS `myStudent`;
# Create student table ：stu
CREATE TABLE stu (
	sid       CHAR(6),
	sname     VARCHAR(50),
	age       INT,
	gender    VARCHAR(50)
);
# Add data ： Show the students stu Add data to 
INSERT INTO stu VALUES('S_1001', 'liuYi', 35, 'male');
INSERT INTO stu VALUES('S_1002', 'chenEr', 15, 'female');
INSERT INTO stu VALUES('S_1003', 'zhangSan', 95, 'male');
INSERT INTO stu VALUES('S_1004', 'liSi', 65, 'female');
INSERT INTO stu VALUES('S_1005', 'wangWu', 55, 'male');
INSERT INTO stu VALUES('S_1006', 'zhaoLiu', 75, 'female');
INSERT INTO stu VALUES('S_1007', 'sunQi', 25, 'male');
INSERT INTO stu VALUES('S_1008', 'zhouBa', 45, 'female');
INSERT INTO stu VALUES('S_1009', 'wuJiu', 85, 'male');
INSERT INTO stu VALUES('S_1010', 'zhengShi', 5, 'female');
INSERT INTO stu VALUES('S_1011', 'xxx', NULL, NULL);
INSERT INTO stu VALUES('S_1012', NULL, 10, 'female');


# Create an employee table ：emp
CREATE TABLE emp(
	empno      INT,
	ename      VARCHAR(50),
	job        VARCHAR(50),
	mgr        INT,
	hiredate   DATE,
	sal        DECIMAL(7,2),
	comm       DECIMAL(7,2),
	deptno     INT
) ;
# Add data ： Report to employees emp Add data to 
INSERT INTO emp VALUES(7369,'SMITH','CLERK',7902,'1980-12-17',800,NULL,20);
INSERT INTO emp VALUES(7499,'ALLEN','SALESMAN',7698,'1981-02-20',1600,300,30);
INSERT INTO emp VALUES(7521,'WARD','SALESMAN',7698,'1981-02-22',1250,500,30);
INSERT INTO emp VALUES(7566,'JONES','MANAGER',7839,'1981-04-02',2975,NULL,20);
INSERT INTO emp VALUES(7654,'MARTIN','SALESMAN',7698,'1981-09-28',1250,1400,30);
INSERT INTO emp VALUES(7698,'BLAKE','MANAGER',7839,'1981-05-01',2850,NULL,30);
INSERT INTO emp VALUES(7782,'CLARK','MANAGER',7839,'1981-06-09',2450,NULL,10);
INSERT INTO emp VALUES(7788,'SCOTT','ANALYST',7566,'1987-04-19',3000,NULL,20);
INSERT INTO emp VALUES(7839,'KING','PRESIDENT',NULL,'1981-11-17',5000,NULL,10);
INSERT INTO emp VALUES(7844,'TURNER','SALESMAN',7698,'1981-09-08',1500,200,30);
INSERT INTO emp VALUES(7876,'ADAMS','CLERK',7788,'1987-05-23',1100,NULL,20);
INSERT INTO emp VALUES(7900,'JAMES','CLERK',7698,'1981-12-03',950,NULL,30);
INSERT INTO emp VALUES(7902,'FORD','ANALYST',7566,'1981-12-03',3000,NULL,20);
INSERT INTO emp VALUES(7934,'MILLER','CLERK',7782,'1982-01-23',1300,NULL,10);

02、 Create student table (stu)、 Employee list (emp) Complete data query operation

#1. Basic query 
#1.1  Inquire about stu All fields in the table (--> That is, query all columns )
SELECT `sid`,`sname`,`age`,`gender` FROM stu;
SELECT * FROM stu;   # When there are too many fields , Query all fields , among * Express all 
#1.2  Check the student list stu The specified column in : Such as query sid、sname Column 
SELECT `sid`,`sname` FROM stu;

#2. Conditions of the query 
/*2.1  Introduction to condition query 
       A conditional query is given at query time where Clause , stay where The following operators and keywords are used in the clause ：
     -->:  = != <> < <= > >=
     -->: between...and
     -->: IN(set)
     -->: IS NULL
     -->: AND
     -->: OR
     -->: NOT
*/
## Query XX record = Query all records of a piece of data 
#2.2  Check gender for female , And older than 50 The record of 
SELECT * FROM `stu` WHERE `gender`='female' AND `age`>50;
#2.3  Check gender for female , And older than 50 Employee number and name 
SELECT `sid`,`sname` FROM `stu` WHERE `gender`='female' AND `age`>50;
#2.4  The student ID is S_1001, Or the name is lisi The record of 
SELECT * FROM stu WHERE `sid`='S_1001' OR `sname`='lisi';
#2.5  The student ID is S_1001,S_1002,S_1003 The record of 
SELECT * FROM stu WHERE `sid`='S_1001' OR `sid`='S_1002' OR `sid`='S_1003';
SELECT * FROM stu WHERE `sid` IN('S_1001','S_1002','S_1003');
#2.6  It's not S_1001,S_1002,S_1003 The record of 
SELECT * FROM stu WHERE `sid`!='S_1001' AND `sid`!='S_1002' AND `sid`!='S_1003';
SELECT * FROM stu WHERE sid NOT IN('S_1001','S_1002','S_1003');
#2.7  Query age is null The record of 
SELECT * FROM stu WHERE `age` IS NULL;
#2.8  The age of inquiry is 20 To 40 Between student records 
SELECT * FROM stu WHERE `age`>=20 AND `age`<=40;
SELECT * FROM stu WHERE `age` BETWEEN 20 AND 40;
#2.9  Check the student records of non male students 
SELECT * FROM stu WHERE `gender`='female';
SELECT * FROM stu WHERE `gender`!='male';
SELECT * FROM stu WHERE `gender`<>'male';
SELECT * FROM stu WHERE NOT `gender`='male';
#2.10  The search name is not null Student records of 
SELECT * FROM stu WHERE `sname` IS NOT NULL;
SELECT * FROM stu WHERE NOT `sname` IS NULL;


#3. Fuzzy query ---LIKE
/*
    When you want to search for names that contain a Students of characters need to use fuzzy query .
    Fuzzy query needs to use keywords ：LIKE
      wildcard ：
        _  Represents any character 
        %  Express arbitrarily 0~n Characters 
*/
#3.1  Search name by 5 Student records in letters 
SELECT * FROM stu WHERE `sname` LIKE '_____';
#3.2  Search name by 5 Letter composition , And No 5 The letters are "i" Student records of 
SELECT * FROM stu WHERE `sname` LIKE '____i';
#3.3  Check the name with “z” The student record at the beginning 
SELECT * FROM stu WHERE `sname` LIKE 'z%';
#3.4  Check the number of 2 The letters are “i” Student records of 
SELECT * FROM stu WHERE `sname` LIKE '_i%';
#3.5  The query name contains “a” The student record of letters 
SELECT * FROM stu WHERE `sname` LIKE '%a%';

#4. Field control query 
#4.1  Remove duplicate records :DISTINCT 
SELECT DISTINCT sal FROM emp;
#4.2  Check the sum of the employee's monthly salary and Commission     Be careful :  A number with NULL Carry out operations , The result is NULL
/*(1)
      because sal and comm Both columns are numeric , So you can add .
      If sal or comm One of the fields in is not a numeric type , Then there will be mistakes .
      for example ： There will be data errors 
	SELECT *,sal+comm FROM emp;
      Be careful ：  A number with NULL Carry out operations , The result is NULL
  (2)
     comm There are many recorded values listed as NULL, Because of anything with NULL The sum is still NULL,
      So the settlement result may appear NULL. Now we use a NULL Convert to value 0 Function of IFNULL：
*/
SELECT *,sal+IFNULL(comm,0) FROM emp;
#4.3  Add an alias to the column name , Use As keyword    As Keywords can be omitted 
SELECT *,sal+IFNULL(comm,0) AS ' The sum of monthly salary and Commission ' FROM emp;
SELECT *,sal+IFNULL(comm,0) ' The sum of monthly salary and Commission ' FROM emp;

#5. Sort  ORDER BY   ASC Ascending ( Default sort )  DESC Descending 
#5.1  Check all student records , Sort by age in ascending order 
SELECT * FROM stu ORDER BY `age` ASC;
#5.2  Check all student records , Sort by age in descending order 
SELECT * FROM stu ORDER BY `age` DESC;
#5.3  Check all employees , Sort by monthly salary in descending order , If the monthly salary is the same , Sort by number in ascending order 
SELECT * FROM emp ORDER BY `sal` DESC,`empno` ASC;
#5.4  Check all employees , Sort by monthly salary in descending order , If the monthly salary is the same , Sort by number in descending order 
SELECT * FROM emp ORDER BY sal DESC,empno DESC;
#5.5  Check that the salary is higher than 1600 Employee records of , And sort in descending order according to salary 
SELECT * FROM emp WHERE sal>1600 ORDER BY sal DESC;

#6. Aggregate functions 
/*
   Aggregate function is a function used for vertical operation 
  （1）COUNT()： The specified column of statistics is not NULL The number of record lines 
  （2）MAX()： Calculate the maximum value of the specified column , If the specified column is of string type , So use string sort operation 
  （3）MIN()： Calculate the minimum value of the specified column , If the specified column is of string type , So use string sort operation 
  （4）SUM(): Calculates the value of the specified column and , If the specified column type is not a numeric type , So the result is 0;
  （5）AVG(): Calculate the average value of the specified column , If the specified column type is not a numeric type , So the result is 0;
*/
#（1）COUNT()： The specified column of statistics is not NULL The number of record lines 
SELECT COUNT(comm) AS ' Number of people with Commission ' FROM emp;
SELECT COUNT(mgr) AS ' Number of employees with leadership ' FROM emp;
SELECT COUNT(*) AS ' Number of data ' FROM emp;
#（2）MAX(): Calculate the maximum value of the specified column , If the specified column is of string type , So use string sort operation 
SELECT MAX(sal) AS ' Maximum wage ' FROM emp;
#（3）MIN(): Calculate the minimum value of the specified column , If the specified column is of string type , So use string sort operation ;
SELECT MIN(sal) AS ' minimum wage ' FROM emp;
#（4）SUM(): Calculates the value of the specified column and , If the specified column type is not a numeric type , So the result is 0;
SELECT SUM(sal) AS ' The sum of employees' wages ' FROM emp;
#（5）AVG(): Calculate the average value of the specified column , If the specified column type is not a numeric type , So the result is 0;
SELECT AVG(sal) AS ' The average wage of the employees ' FROM emp;

# Inquire about emp The monthly salary in the table is more than 2500 The number of people 
SELECT COUNT(*) AS ' The monthly salary is greater than 2500 The number of people ' FROM emp WHERE sal>2500; 
# The sum of monthly salary and commission is greater than 2500 The number of yuan 
SELECT COUNT(sal+IFNULL(comm,0)) AS ' The sum of monthly salary and commission is greater than 2500 The number of yuan ' FROM emp WHERE sal+IFNULL(comm,0)>2500;
# Check the monthly salary of all employees + Commission and 
SELECT SUM(sal+IFNULL(comm,0)) AS ' Monthly salary of all employees + Commission and ' FROM emp;