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MySQL (2) -- simple query, conditional query

2022-07-05 23:13:00 【Dutkig】

Simple query

The database file we used is still the one used in the previous section test.sql SQL Statement is case insensitive , It can also be mixed , But be careful to use half width semicolon ; ending , Let's start with a simple query ：

Query a field

select  Field name  from  Table name ;

Be careful ：select and from It's all keywords , Field names and table names are identifiers .
for example ： Query department name

mysql> select dname from dept;
	+------------+
	| dname      |
	+------------+
	| ACCOUNTING |
	| RESEARCH   |
	| SALES      |
	| OPERATIONS |
	+------------+
	4 rows in set (0.00 sec)

Query two or more fields
—— Separated by commas
for example ： Query department number and department name

select deptno,dname from dept;
	+--------+------------+
	| deptno | dname      |
	+--------+------------+
	|     10 | ACCOUNTING |
	|     20 | RESEARCH   |
	|     30 | SALES      |
	|     40 | OPERATIONS |
	+--------+------------+

Query all fields

select * from dept;
	+--------+------------+----------+
	| DEPTNO | DNAME      | LOC      |
	+--------+------------+----------+
	|     10 | ACCOUNTING | NEW YORK |
	|     20 | RESEARCH   | DALLAS   |
	|     30 | SALES      | CHICAGO  |
	|     40 | OPERATIONS | BOSTON   |
	+--------+------------+----------+

Disadvantages of this approach ：

Low efficiency , Because it will put * Convert to all fields .
Poor readability .
In actual development, it is not recommended that , The actual development proposal indicates the required fields .

Alias the columns of the query ——as keyword

mysql> select deptno,dname as deptname from dept;
	+--------+------------+
	| deptno | deptname   |
	+--------+------------+
	|     10 | ACCOUNTING |
	|     20 | RESEARCH   |
	|     30 | SALES      |
	|     40 | OPERATIONS |
	+--------+------------+

Be careful ： Just display the query result column name as deptname, The original table column name is still called ：dname
remember ：select Statement will never be modified .（ Because it is only responsible for querying ）

as Keywords can also be omitted

mysql> select deptno,dname deptname from dept;

Let's assume that when you use an alias , There are spaces in the alias , What do I do ？
mysql> select deptno,dname dept name from dept;
DBMS See such a statement , Conduct SQL Statement compilation , It's not grammatical , Compiler error . How to solve ？

select deptno,dname 'dept name' from dept; // Charizing Operator 
select deptno,dname "dept name" from dept; // quotation marks 
	+--------+------------+
	| deptno | dept name  |
	+--------+------------+
	|     10 | ACCOUNTING |
	|     20 | RESEARCH   |
	|     30 | SALES      |
	|     40 | OPERATIONS |
	+--------+------------+

Be careful ： In all databases , Strings are uniformly enclosed in single quotes , Single quotation marks are standard , Double quotation marks in oracle Not in the database . But in mysql Can be used in . Again ： All strings in the database are enclosed in single quotes . It's standard , Double quotation marks are not standard .

Columns are involved in mathematical operations
Calculate the annual salary of employees ：sal*12

mysql> select ename,sal from emp;
	+--------+---------+
	| ename  | sal     |
	+--------+---------+
	| SMITH  |  800.00 |
	| ALLEN  | 1600.00 |
	| WARD   | 1250.00 |
	| JONES  | 2975.00 |
	| MARTIN | 1250.00 |
	| BLAKE  | 2850.00 |
	| CLARK  | 2450.00 |
	| SCOTT  | 3000.00 |
	| KING   | 5000.00 |
	| TURNER | 1500.00 |
	| ADAMS  | 1100.00 |
	| JAMES  |  950.00 |
	| FORD   | 3000.00 |
	| MILLER | 1300.00 |
	+--------+---------+
mysql> select ename,sal*12 from emp; 
	+--------+----------+
	| ename  | sal*12   |
	+--------+----------+
	| SMITH  |  9600.00 |
	| ALLEN  | 19200.00 |
	| WARD   | 15000.00 |
	| JONES  | 35700.00 |
	| MARTIN | 15000.00 |
	| BLAKE  | 34200.00 |
	| CLARK  | 29400.00 |
	| SCOTT  | 36000.00 |
	| KING   | 60000.00 |
	| TURNER | 18000.00 |
	| ADAMS  | 13200.00 |
	| JAMES  | 11400.00 |
	| FORD   | 36000.00 |
	| MILLER | 15600.00 |
	+--------+----------+

Conclusion ： Fields can use mathematical expressions

mysql> select ename,sal*12 as yearsal from emp; // names 
	+--------+----------+
	| ename  | yearsal  |
	+--------+----------+
	| SMITH  |  9600.00 |
	| ALLEN  | 19200.00 |
	| WARD   | 15000.00 |
	| JONES  | 35700.00 |
	| MARTIN | 15000.00 |
	| BLAKE  | 34200.00 |
	| CLARK  | 29400.00 |
	| SCOTT  | 36000.00 |
	| KING   | 60000.00 |
	| TURNER | 18000.00 |
	| ADAMS  | 13200.00 |
	| JAMES  | 11400.00 |
	| FORD   | 36000.00 |
	| MILLER | 15600.00 |
	+--------+----------+

mysql> select ename,sal*12 as ' Annual salary ' from emp; // The alias is Chinese , In single quotation marks .
	+--------+----------+
	| ename  |  Annual salary         |
	+--------+----------+
	| SMITH  |  9600.00 |
	| ALLEN  | 19200.00 |
	| WARD   | 15000.00 |
	| JONES  | 35700.00 |
	| MARTIN | 15000.00 |
	| BLAKE  | 34200.00 |
	| CLARK  | 29400.00 |
	| SCOTT  | 36000.00 |
	| KING   | 60000.00 |
	| TURNER | 18000.00 |
	| ADAMS  | 13200.00 |
	| JAMES  | 11400.00 |
	| FORD   | 36000.00 |
	| MILLER | 15600.00 |
	+--------+----------+

Conditions of the query

What is a conditional query ？—— Not all the data in the table are found out . It is found that the qualified .
Grammar format ：

select
		 Field 1, Field 2, Field 3....
	from 
		 Table name 
	where
		 Conditions ;

Types of conditions ：
① = be equal to

 Query salary equals 800 Employee's name and number ？
	select empno,ename from emp where sal = 800;
	
 Inquire about SMITH Your number and salary ？
	select empno,sal from emp where ename = 'SMITH'; // String using single quotes

② <> or != It's not equal to

 Query salary is not equal to 800 Employee's name and number ？
	select empno,ename from emp where sal != 800;
	select empno,ename from emp where sal <> 800; //  An unequal sign consisting of the less than sign and the greater than sign

③ Greater than or equal to

 Query salary less than 2000 Employee's name and number ？
	mysql> select empno,ename,sal from emp where sal < 2000;
	+-------+--------+---------+
	| empno | ename  | sal     |
	+-------+--------+---------+
	|  7369 | SMITH  |  800.00 |
	|  7499 | ALLEN  | 1600.00 |
	|  7521 | WARD   | 1250.00 |
	|  7654 | MARTIN | 1250.00 |
	|  7844 | TURNER | 1500.00 |
	|  7876 | ADAMS  | 1100.00 |
	|  7900 | JAMES  |  950.00 |
	|  7934 | MILLER | 1300.00 |
	+-------+--------+---------+

 Query salary less than or equal to 3000 Employee's name and number ？
	select empno,ename,sal from emp where sal <= 3000;

 Query salary greater than 3000 Employee's name and number ？
	select empno,ename,sal from emp where sal > 3000;

 Query salary greater than or equal to 3000 Employee's name and number ？
	select empno,ename,sal from emp where sal >= 3000;

④between … and …. Between two values （ When using, it must be small on the left and large on the right , Otherwise, we can't find ） , Equate to >= and <=

 Query salary in 2450 and 3000 Employee information between ？ Include 2450 and 3000
	 The first way ：>= and <= （and Is and means .）
		select empno,ename,sal from emp where sal >= 2450 and sal <= 3000;
		+-------+-------+---------+
		| empno | ename | sal     |
		+-------+-------+---------+
		|  7566 | JONES | 2975.00 |
		|  7698 | BLAKE | 2850.00 |
		|  7782 | CLARK | 2450.00 |
		|  7788 | SCOTT | 3000.00 |
		|  7902 | FORD  | 3000.00 |
		+-------+-------+---------+
	 The second way ：between … and …
		select 
			empno,ename,sal 
		from 
			emp 
		where 
			sal between 2450 and 3000;

Be careful ： between and It's a closed interval , Include values at both ends .
⑤ is null by null（is not null Not empty ）

 Query which employees' allowances / Subsidy is null？
	mysql> select empno,ename,sal,comm from emp where comm = null;
	Empty set (0.00 sec)

	mysql> select empno,ename,sal,comm from emp where comm is null;
	+-------+--------+---------+------+
	| empno | ename  | sal     | comm |
	+-------+--------+---------+------+
	|  7369 | SMITH  |  800.00 | NULL |
	|  7566 | JONES  | 2975.00 | NULL |
	|  7698 | BLAKE  | 2850.00 | NULL |
	|  7782 | CLARK  | 2450.00 | NULL |
	|  7788 | SCOTT  | 3000.00 | NULL |
	|  7839 | KING   | 5000.00 | NULL |
	|  7876 | ADAMS  | 1100.00 | NULL |
	|  7900 | JAMES  |  950.00 | NULL |
	|  7902 | FORD   | 3000.00 | NULL |
	|  7934 | MILLER | 1300.00 | NULL |
	+-------+--------+---------+------+
	10 rows in set (0.00 sec)

Be careful ： In the database null You can't use the equal sign to measure . Need to use is null, Because in the database null Means nothing , It's not a value , So you can't use the equal sign to measure .

 Query which employees' allowances / The subsidy is not null？
	select empno,ename,sal,comm from emp where comm is not null;
	+-------+--------+---------+---------+
	| empno | ename  | sal     | comm    |
	+-------+--------+---------+---------+
	|  7499 | ALLEN  | 1600.00 |  300.00 |
	|  7521 | WARD   | 1250.00 |  500.00 |
	|  7654 | MARTIN | 1250.00 | 1400.00 |
	|  7844 | TURNER | 1500.00 |    0.00 |
	+-------+--------+---------+---------+

⑥and also

 The job search is MANAGER And the salary is higher than 2500 Employee information ？
	select 
		empno,ename,job,sal 
	from 
		emp 
	where 
		job = 'MANAGER' and sal > 2500;
	
	+-------+-------+---------+---------+
	| empno | ename | job     | sal     |
	+-------+-------+---------+---------+
	|  7566 | JONES | MANAGER | 2975.00 |
	|  7698 | BLAKE | MANAGER | 2850.00 |
	+-------+-------+---------+---------+

⑦or perhaps

 The job search is MANAGER and SALESMAN The employees' ？
	select empno,ename,job from emp where job = 'MANAGER';
	select empno,ename,job from emp where job = 'SALESMAN';

	select 
		empno,ename,job
	from
		emp
	where 
		job = 'MANAGER' or job = 'SALESMAN';
		
	+-------+--------+----------+
	| empno | ename  | job      |
	+-------+--------+----------+
	|  7499 | ALLEN  | SALESMAN |
	|  7521 | WARD   | SALESMAN |
	|  7566 | JONES  | MANAGER  |
	|  7654 | MARTIN | SALESMAN |
	|  7698 | BLAKE  | MANAGER  |
	|  7782 | CLARK  | MANAGER  |
	|  7844 | TURNER | SALESMAN |
	+-------+--------+----------+

and and or At the same time , Is there a priority problem ？

 Query salary greater than 2500, And the department number is 10 or 20 Employees of the Department ？
	select 
		*
	from
		emp
	where
		sal > 2500 and deptno = 10 or deptno = 20;

Analyze the problem of the above sentence ？—— and Priority ratio or high . The above statement will be executed first and, And then execute or.
So what does the above sentence mean ？
Find out that the salary is greater than 2500 And the department number is 10 The employees' , perhaps 20 Find out all the employees in the Department .

select 
			*
		from
			emp
		where
			sal > 2500 and (deptno = 10 or deptno = 20);

and and or At the same time ,and High priority . If you want to let or Execute first , Need to add “ parentheses ”, If you're not sure about priorities , Just add parentheses .

⑧in contain , It's equivalent to more than one or （not in Not in this range ）

 The job search is MANAGER and SALESMAN The employees' ？
		select empno,ename,job from emp where job = 'MANAGER' or job = 'SALESMAN';
		select empno,ename,job from emp where job in('MANAGER', 'SALESMAN');
		+-------+--------+----------+
		| empno | ename  | job      |
		+-------+--------+----------+
		|  7499 | ALLEN  | SALESMAN |
		|  7521 | WARD   | SALESMAN |
		|  7566 | JONES  | MANAGER  |
		|  7654 | MARTIN | SALESMAN |
		|  7698 | BLAKE  | MANAGER  |
		|  7782 | CLARK  | MANAGER  |
		|  7844 | TURNER | SALESMAN |
		+-------+--------+----------+

Be careful ：in Not an interval .in Followed by the specific value .

 Query salary is 800 and 5000 Employee information ？
		select ename,sal from emp where sal = 800 or sal = 5000;
		select ename,sal from emp where sal in(800, 5000); // This doesn't mean 800 To 5000 Find out .
		+-------+---------+
		| ename | sal     |
		+-------+---------+
		| SMITH |  800.00 |
		| KING  | 5000.00 |
		+-------+---------+
		select ename,sal from emp where sal in(800, 5000, 3000);

		// not in  Represents data that is not among these values .
		select ename,sal from emp where sal not in(800, 5000, 3000);
		+--------+---------+
		| ename  | sal     |
		+--------+---------+
		| ALLEN  | 1600.00 |
		| WARD   | 1250.00 |
		| JONES  | 2975.00 |
		| MARTIN | 1250.00 |
		| BLAKE  | 2850.00 |
		| CLARK  | 2450.00 |
		| TURNER | 1500.00 |
		| ADAMS  | 1100.00 |
		| JAMES  |  950.00 |
		| MILLER | 1300.00 |
		+--------+---------+