A query for a field

Query the employee's name

mysql> select ename from emp;
+--------+
| ename  |
+--------+
| SMITH  |
| ALLEN  |
| WARD   |
| JONES  |
| MARTIN |
| BLAKE  |
| CLARK  |
| SCOTT  |
| KING   |
| TURNER |
| ADAMS  |
| JAMES  |
| FORD   |
| MILLER |
+--------+

Select statement is followed by field name, select is the key word, and space is used between select and field name, from represents the table to be queried, and space is used between fields.

Two Queries for Multiple Fields

Query the employee's number and name

mysql> select empno, ename from emp;
+-------+--------+
| empno | ename  |
+-------+--------+
|  7369 | SMITH  |
|  7499 | ALLEN  |
|  7521 | WARD   |
|  7566 | JONES  |
|  7654 | MARTIN |
|  7698 | BLAKE  |
|  7782 | CLARK  |
|  7788 | SCOTT  |
|  7839 | KING   |
|  7844 | TURNER |
|  7876 | ADAMS  |
|  7900 | JAMES  |
|  7902 | FORD   |
|  7934 | MILLER |
+-------+--------+

Query multiple fields, select fields using comma interval, the last field, that is, the field in front of from can not use commas.

Three queries for all fields

mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | 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 | 1981-05-23 | 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 |
+-------+--------+-----------+------+------------+---------+---------+--------+

select * from emp, although simple, but the * number is not very clear, it is recommended to query all fields and write the relevant fields after the select statement. In the future when java connects to the database, it is necessary to write the SQL statement in the java program. The SQL statement written at this time is not recommended to use select. * In this form, it is recommended to specify the fields so that they are readable.

4. Calculating the Annual Salary of Employees

List the employee's number, name and annual salary

mysql> select empno, ename, sal*12 from emp;
+-------+--------+----------+
| empno | ename  | sal*12   |
+-------+--------+----------+
|  7369 | SMITH  |  9600.00 |
|  7499 | ALLEN  | 19200.00 |
|  7521 | WARD   | 15000.00 |
|  7566 | JONES  | 35700.00 |
|  7654 | MARTIN | 15000.00 |
|  7698 | BLAKE  | 34200.00 |
|  7782 | CLARK  | 29400.00 |
|  7788 | SCOTT  | 36000.00 |
|  7839 | KING   | 60000.00 |
|  7844 | TURNER | 18000.00 |
|  7876 | ADAMS  | 13200.00 |
|  7900 | JAMES  | 11400.00 |
|  7902 | FORD   | 36000.00 |
|  7934 | MILLER | 15600.00 |
+-------+--------+----------+

Operators can be used in select statements.

Fifth, rename the queried fields

List employee numbers, names, annual salaries

mysql> select empno, ename, sal*12 as 'Annual salary' from emp;
+-------+--------+----------+
| empno | ename  | Annual salary        |
+-------+--------+----------+
|  7369 | SMITH  |  9600.00 |
|  7499 | ALLEN  | 19200.00 |
|  7521 | WARD   | 15000.00 |
|  7566 | JONES  | 35700.00 |
|  7654 | MARTIN | 15000.00 |
|  7698 | BLAKE  | 34200.00 |
|  7782 | CLARK  | 29400.00 |
|  7788 | SCOTT  | 36000.00 |
|  7839 | KING   | 60000.00 |
|  7844 | TURNER | 18000.00 |
|  7876 | ADAMS  | 13200.00 |
|  7900 | JAMES  | 11400.00 |
|  7902 | FORD   | 36000.00 |
|  7934 | MILLER | 15600.00 |
+-------+--------+----------+

as can be omitted, renamed strings to add single quotation marks, especially in Chinese, must be added, if it is English characters, you can not add quotation marks.