SQL statements are the language for dealing with databases.
SQL statements themselves are not case sensitive, but usually
Capitalize keywords and non-keywords
diff to increase readability.
SELECT SYSDATE FROM dual
DDL statement
DDL statements are used to add, delete, and modify database objects
Database objects: tables, views, indexes, sequences
Create table:
CREATE TABLE employee(
id NUMBER(4),
name VARCHAR2(20),
gender CHAR(1),
birth DATE,
salary NUMBER(6,2),
job VARCHAR2(30),
deptno NUMBER(2)
)
View table structure
DESC employee_xxxx
In the database, no matter what type the field is,
The default values are all NULL.
When we insert data into a table, some
If the field does not have a given value, the field will be
Default value is inserted.
You can use the DEFAULT keyword when creating a table
Defines default values individually for the specified fields.
In the database, string literals are used
are single quotes, and the string content is case sensitive
written
delete table:
DROP TABLE employee
CREATE TABLE employee(
id NUMBER(4),
name VARCHAR2(20),
gender CHAR(1) DEFAULT 'M',
birth DATE,
salary NUMBER(6,2),
job VARCHAR2(30),
deptno NUMBER(2)
);
Modify table
1: Modify the table name
RENAME employee TO myemp
DESC myemp
2: Modify the table structure
2.1: Add fields
Adding fields to a table can only be added to the table's
Finally, you cannot insert between existing fields
a field.
ALTER TABLE myemp
ADD(
hiredate DATE DEFAULT SYSDATE
)
DESC myemp
2.2: Delete existing fields in the table
ALTER TABLE myemp
DROP(hiredate)
DESC myemp
2.3: Modify existing fields
Modify the table structure as far as possible without in the table
data time. When there is already data in the table
When modifying the field, you should pay attention: try not to modify
Field type, the length should not be reduced if modified
Because shrinking may result in a
Failure due to length conflict
ALTER TABLE myemp
MODIFY(
job VARCHAR2(40) DEFAULT 'CLERK'
)
DESC myemp
DML statement
1: INSERT statement
for inserting data into the table
When inserting data, you can not specify the field, if you do not specify
This is a full column insert, after VALUES all
The values corresponding to the fields are all specified.
It is recommended to specify fields to increase readability.
INSERT INTO myemp
(id, name, job, salary)
VALUES
(1001, 'rose', 'PROGRAMMER', 5500)
SELECT * FROM myemp
Insert date is recommended
TO_DATE() function for
INSERT INTO myemp
(id,name,birth,deptno)
VALUES
(2,'jack',
TO_DATE('1992-02-04',
'YYYY-MM-DD'),20)
SELECT * FROM myemp
2: Modify the data in the table
UPDATE statement
WHERE is usually used in UPDATE statements
Add filter conditions so that only the conditions will be met
If the record is not added, the entire record will be modified.
All records in the table are modified!
UPDATE myemp
SET gender='F',id=1002
WHERE name='jack'
3: delete statement
DELETE statement
The delete statement also needs to use WHERE, otherwise it is
Clear table operation!
DELETE FROM myemp
WHERE name='rose'
SELECT * FROM myemp