1. Classification of SQL statements
Learning MySQL is mainly to learn general SQL statements, then SQL statements include additions, deletions, modifications, and queries. How are SQL statements classified?
DQL (Data Query Language): Query statements, all select statements are DQL.
DML (Data Manipulation Language): insert delete update, to add, delete and modify the data in the table.
DDL (Data Definition Language): create drop alter, additions, deletions and changes to the table structure.
TCL (Transaction Control Language): Commit commits the transaction, rollback rolls back the transaction. (T in TCL is Transaction)
DCL (Data Control Language): grant authorization, revoke, etc.
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Note: The case of the database used in this page are shown in the following end :
2. Simple query
Operating table
mysql> select * from emp;
EMPNO | ENAME | JOB | MGR | HIREDATE | SHALL | COMM | DEPTNO |
---|---|---|---|---|---|---|---|
7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 |
7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 |
7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 |
7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 |
7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 |
7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 |
7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
The field names of each field are employee number, employee name, employee position, employee's superior leader number, employee employment date, employee salary, employee allowance, employee department number.
Syntax format:
select field name 1, field name 2, field name 3,... from table name;
Tips:
1. Any sql statement ends with ";".
2. The sql statement is not case sensitive.
select ename, empno from emp;
Note: If the number of displays is too large, or the number of query data displayed is not important, the screenshot generally only captures part of it.
Query the employee's annual salary (fields can participate in mathematical operations.)
select ename,sal*12 from emp;
Rename the column of the query result?
select ename,sal * 12 as yearsal from emp;
Chinese in the alias?
select ename, sal * 12 as annual salary from emp; // Wrong
select ename, sal * 12 as'annual salary' from emp;
Note: The standard SQL statement requires the string to be enclosed in single quotes. Although mysql supports double quotes, try not to use them.
Can the as keyword be omitted?
select empno,ename,sal * 12 yearsal from emp;
query all fields?
select * from emp; It
is not recommended to use in actual development, and the efficiency is low .
Note: The database bjpowernode code used by MySQL in this section is as follows, you can run it directly to generate the database file
DROP TABLE IF EXISTS EMP;
DROP TABLE IF EXISTS DEPT;
DROP TABLE IF EXISTS SALGRADE;
CREATE TABLE DEPT
(DEPTNO int(2) not null ,
DNAME VARCHAR(14) ,
LOC VARCHAR(13),
primary key (DEPTNO)
);
CREATE TABLE EMP
(EMPNO int(4) not null ,
ENAME VARCHAR(10),
JOB VARCHAR(9),
MGR INT(4),
HIREDATE DATE DEFAULT NULL,
SAL DOUBLE(7,2),
COMM DOUBLE(7,2),
primary key (EMPNO),
DEPTNO INT(2)
)
;
CREATE TABLE SALGRADE
( GRADE INT,
LOSAL INT,
HISAL INT );
INSERT INTO DEPT ( DEPTNO, DNAME, LOC ) VALUES (
10, 'ACCOUNTING', 'NEW YORK');
INSERT INTO DEPT ( DEPTNO, DNAME, LOC ) VALUES (
20, 'RESEARCH', 'DALLAS');
INSERT INTO DEPT ( DEPTNO, DNAME, LOC ) VALUES (
30, 'SALES', 'CHICAGO');
INSERT INTO DEPT ( DEPTNO, DNAME, LOC ) VALUES (
40, 'OPERATIONS', 'BOSTON');
commit;
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7369, 'SMITH', 'CLERK', 7902, '1980-12-17'
, 800, NULL, 20);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20'
, 1600, 300, 30);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7521, 'WARD', 'SALESMAN', 7698, '1981-02-22'
, 1250, 500, 30);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7566, 'JONES', 'MANAGER', 7839, '1981-04-02'
, 2975, NULL, 20);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28'
, 1250, 1400, 30);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7698, 'BLAKE', 'MANAGER', 7839, '1981-05-01'
, 2850, NULL, 30);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7782, 'CLARK', 'MANAGER', 7839, '1981-06-09'
, 2450, NULL, 10);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7788, 'SCOTT', 'ANALYST', 7566, '1987-04-19'
, 3000, NULL, 20);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7839, 'KING', 'PRESIDENT', NULL, '1981-11-17'
, 5000, NULL, 10);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7844, 'TURNER', 'SALESMAN', 7698, '1981-09-08'
, 1500, 0, 30);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7876, 'ADAMS', 'CLERK', 7788, '1987-05-23'
, 1100, NULL, 20);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7900, 'JAMES', 'CLERK', 7698, '1981-12-03'
, 950, NULL, 30);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7902, 'FORD', 'ANALYST', 7566, '1981-12-03'
, 3000, NULL, 20);
INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,
DEPTNO ) VALUES (
7934, 'MILLER', 'CLERK', 7782, '1982-01-23'
, 1300, NULL, 10);
commit;
INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES (
1, 700, 1200);
INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES (
2, 1201, 1400);
INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES (
3, 1401, 2000);
INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES (
4, 2001, 3000);
INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES (
5, 3001, 9999);
commit;