Oracle database knowledge points that cannot be learned (II)

SQL Query and SQL function

1.SQl Supported commands ：

Data definition language （DDL）:create,alter,drop

Data manipulation language （DML）:insert,delete,update,select

Data control language （DCL）:grant,revoke

Transaction control language （TCL）:commit,savepoint,rollback

2.Oracle data type

character , The number , date ,RAW,LOB

Character

char:1-2000 Fixed length characters of bytes

varchar2:1-4000 Variable length characters of bytes

long:2GB Variable length character of

  Be careful ： At most one column in a table can be long type

Long Columns cannot define unique constraints or primary key constraints

long Cannot create index on column

Procedures or stored procedures are not acceptable long Parameters of type .

Numerical type

number: Highest accuracy 38 position

Date time type

date: Accurate to ss

timestamp: The second value is accurate to the decimal point 6 position

function sysdate,systimestamp Return the current system date , Time and time zone .

Change the display of time

alter session set nls_date_language=’american’;

alter session set nls_date_format=’yyyy-mm-dd’;

Oracle Pseudo columns in , Like a table column , But it is not stored in the table , Pseudo columns can be queried , But you can't insert 、 Update and modify their values , Common pseudo columns ：rowid and rownum.

rowid: The storage address of the row in the table , It can uniquely identify a row in the database , You can use this column to quickly locate rows in the table .

rownum: The query returns the sequence number of the row in the result set , You can use it to limit the number of rows returned by the query .

3. Data definition language

Commands for manipulating tables

create table

alter table

truncate table

drop table

The command to modify the table

alter table stu_table rename to stu_tbl;-- Modify the name of the table 

alter table stu_tbl rename column stu_sex to sex;-- Change column names 

alter table stu_tbl add (stu_age number);-- Add new column 

alter table stu_tbl drop(sex);-- Delete column 

alter table stu_tbl modify(stu_sex varchar2(2));-- Change the data type of the column 

alter table stu_tbl add constraint pk_stu_tbl primary key(id);-- Adding constraints 

4. Data manipulation language

select,update,delete,insert

Create a table from an existing table

create table stu_tbl_log as select id,stu_name,stu_age from stu_tbl;-- Select a row without repetition 

select distinct stu_name from stu_tbl;-- Insert records from other tables 

insert into stu_tbl_log select id,stu_name,stu_age from stu_tbl;

5. Data control language

grant,revoke

6. Transaction control language

commit,savepoint,rollback

7.SQL The operator

arithmetic operator :+-*/

Comparison operator :=,!=,<>,>,<,>=,<=,between-and,in,like,is null etc.

Logical operators :and,or,not

Set operators :union,union all,intersect,minus

Join operators :||

Example ：

select (3+2)/2 from dual;-- arithmetic operator , result ：2.5

select * from stu_tbl where stu_age>=20;-- Comparison operator 

select * from stu_tbl where stu_name like '%a%';-- Comparison operator ：like

select * from stu_tbl where stu_name like 'a___';-- Comparison operator ：like

select * from stu_tbl where stu_age in(20,30);-- Comparison operator ：in

select * from stu_tbl where stu_age between 20 and 30;-- Comparison operator ：between

select stu_name from stu_tbl union all

select stu_name from stu_tbl_log;-- Set operators ：union all
select stu_name from stu_tbl union

select stu_name from stu_tbl_log;-- Set operators ：union
select stu_name from stu_tbl intersect

select stu_name from stu_tbl_log;-- Set operators ：intersect
select stu_name from stu_tbl minus

select stu_name from stu_tbl_log;-- Set operators ：minus

It can be seen from it that ：

minus Is to get the data unique to the first table

intersect It is to obtain the data in both tables

union Is to integrate the data of two tables , All of them are displayed only once

union all It is pure data integration of two tables

select id,stu_name||' '||stu_sex as name_sex,stu_age

from stu_tbl;-- Join operators ||

8.SQL function

One line function ： Only one value is returned for each row queried from the table , Can appear in select Clause ,where clause

Date function

Number function

Character functions

Conversion function ：ToChar(),ToDate(),ToNumber()

Other functions :

Nvl(exp1,exp2): Expression 1 is null when , Return expression two

Nvl2(exp1,exp2,exp3): Expression 1 is null Returns expression three , Otherwise, it returns expression two

Nullif(exp1,exp2): When two expressions are equal , return null, Otherwise, return the expression one

Group function ： Return based on a set of rows

Avg,Min,Max,Sum,Count

Group by,having

Analysis function

Row_number,rank,dense_rank

Example ：

select u.user_name,sum(oi.order_num*oi.order_price) as total,row_number() over (order by sum(oi.order_num*oi.order_price) desc) as sort from order_item_tbl

  oi,user_tbl  u,order_tbl  o where oi.order_id = o.id and o.user_id = u.id group by u.user_name;