Databases: Fuzzy queries, views, sequences, indexes

1. Fuzzy query

Keyword "like"

% stands for any character

_ Represents a character

Like '%A ' is the last person in the name of a.

Like ' A% ' is the first character in the name of a person

Select  from where  like _b (%b)   // Query the second character in the Test1 column in the test table is B data (contains B data)

2. View

Benefits:

1) Restricting access to the database
2) Simplified Query
3) Maintain the independence of data
4) Different views can be created for the same data
5) You can delete a view without affecting the data

Create [or replace] View  as (Data to build the view)    // as after can be implemented by the SELECT statement, delete the View drop V_test Select *  from User_views                              // View data dictionary (the specific contents of the user's view) User_views is the system's table

3. Sequence

The method of constructing the sequence, which is usually used to assign a value to the primary key

CreateSequence Seq_test1//Create sequence Increment by 1                   //1start per increase with   1                   //starting position 1maxvalueTen                      //maximum value is 10minvalue1                      //minimum value is 1cycle/Nocycle//Cycle/do not cycle the cache5                          //In-memory cache of 5

Select  from dual    // querying the current value in the test1 sequence Select from dual    // Query the next value of the test1 sequence (note that viewing the next value is equivalent to using                              it) dual // system table, which contains various information about the sequence

4. Index

Improve the speed of retrieving and querying data

Create Index  on desc)  // CREATE INDEX statement (details of self-Baidu, here is not deep) Select *  from user_indexes                   // View Index

Databases: Fuzzy queries, views, sequences, indexes