Summary of basic MySQL commands and common commands _ MySQL

Summary of basic MySQL commands and common commands bitsCN.com

Start: net start mysql;
Enter: mysql-u root-p/mysql-h localhost-u root-p databaseName;
List databases: show databases;
Select database: use databaseName;
List tables: show tables;
Show columns from tableName;
Create a database: source fileName.txt;
Matching character: Wildcard _ can be used to represent any character, and % represents any string;
Add a field: alter table tabelName add column fieldName dateType;
Add multiple fields: alter table tabelName add column fieldName1 dateType, add columns fieldName2 dateType;
Multi-line command input: note that words cannot be broken; when data is inserted or changed, the field strings cannot be expanded into multiple rows; otherwise, the hard press enter will be stored in the data;
Add an administrator account: grant all on *. * to user @ localhost identified by "password ";
After each statement is entered, enter the plus sign ';' at the end, or add '/g;
Query Time: select now ();
Query the current user: select user ();
Query the database version: select version ();
Query the currently used database: select database ();

1. delete the students Data table in the student_course database:
Rm-f student_course/students .*

2. back up the database: (back up the database test)
Mysqldump-u root-p test> c:/test.txt
Backup table: (back up the mytable table under the test database)
Mysqldump-u root-p test mytable> c:/test.txt
Import the backup data to the database: (import back to the test database)
Mysql-u root-p test

3. create a temporary table: (create a temporary table zengchao)
Create temporary table zengchao (name varchar (10 ));

4. to create a table, first determine whether the table exists.
Create table if not exists students (......);

5. copy the table structure from an existing table
Create table table2 select * from table1 where 1 <> 1;

6. copy a table
Create table table2 select * from table1;

7. rename the table
Alter table table1 rename as table2;

8. modify the column type
Alter table table1 modify id int unsigned; // modify the column id type to int unsigned
Alter table table1 change id sid int unsigned; // modify the column id name to sid and the attribute to int unsigned

9. create an index
Alter table table1 add index ind_id (id );
Create index ind_id on table1 (id );
Create unique index ind_id on table1 (id); // create a unique index

10. delete an index
Drop index idx_id on table1;
Alter table table1 drop index ind_id;

11. combine characters or multiple columns (link the column id with ":" And column name and "=)
Select concat (id, ':', name, '=') from students;

12. limit (10 to 20 items are selected) <第一个记录集的编号是0>
Select * from students order by id limit 9, 10;

13. functions not supported by MySQL
Transaction, view, foreign key and reference integrity, stored procedure and trigger

14. MySQL uses the index operator symbol
<, <=, >=, >,=, Between, in, like without the start of % or _

15. disadvantages of using indexes
1) slowing down the speed of adding, deleting, and modifying data;
2) occupied disk space;
3) increase the burden on the query optimizer;
When the query optimizer generates an execution plan, it will consider indexes. too many indexes will increase the workload for the query optimizer, resulting in the inability to select the optimal query solution;

16. analyze index efficiency
Method: Add explain before a general SQL statement;
Meaning of analysis results:
1) table: The table name;
2) type: connection type (ALL/Range/Ref ). Among them, ref is the most ideal;
3) possible_keys: query the available index names;
4) key: actually used index;
5) key_len: the length (in bytes) of the used part in the index );
6) ref: displays the column name or "const" (I don't understand what it means );
7) rows: displays the number of rows that MySQL considers to be scanned before finding the correct results;
8) extra: MySQL recommendations;

17. Use a short fixed-length column
1) try to use a shorter data type;
2) use the fixed-length data type as much as possible;
A) replace varchar with char, and process data with a fixed length faster than that with a variable length;
B) for tables that are frequently modified, disks are easy to form fragments, which affects the overall performance of the database;
C) in case of a data table crash, it is easier to reconstruct a table with a fixed length of data rows. When a data row with a fixed length is used, the start position of each record is a multiple of the fixed record length, which can be easily detected. However, it is not necessary to use a variable length data row;
D) for MyISAM data tables, although converting to a fixed-length data column can improve performance, it occupies a large space;

18. use not null and enum
Define the column as not null as much as possible, so that data can be generated faster and less space is required. In addition, MySQL does not need to check whether there is a special case, that is, the null value, optimize the query;
If a column contains only a limited number of specific values, such as gender, validity, or enrollment year, you should consider converting it to the value of the enum column, mySQL processes faster, because all enum values are represented by an identifier value in the system;

19. use optimize table
For tables that are frequently modified, fragments are easily generated, so that more disk blocks must be read during database query to reduce query performance. All tables with variable lengths have disk fragmentation issues. this problem is more prominent for blob data types because of its large size changes. You can use optimize table to organize fragments to ensure that the database performance is not degraded and optimize the data tables affected by fragments. Optimize table can be used for MyISAM and BDB data tables. In fact, any fragment method uses mysqldump to store data tables, and then creates a new data table using the converted files;

20. use procedure analyze ()
Procedure analyze () can be used to display the best type of recommendations. it is easy to use. add procedure analyze () after the select statement. for example:
Select * from students procedure analyze ();
Select * from students procedure analyze (16,256 );
The second statement requires that procedure analyze () should not contain more than 16 values or enum types containing more than 256 bytes. if there is no limit, the output may be very long;

21. use query cache
1) How to query the cache:
When a select statement is executed for the first time, the server remembers the text content and query results of the query and stores them in the cache. when this statement is executed for the next time, the server returns the results directly from the cache; when a data table is updated, any cache query of the data table becomes invalid and is discarded.
2) configure cache parameters:
Variable: query_cache _ type, the operation mode of the query cache. There are 3 moderate modes, 0: No cache; 1: cache query, unless it starts with select SQL _no_cache; 2: Only queries starting with select SQL _cache are cached as needed; query_cache_size: set the maximum result set size of the query cache, which is greater than this value and will not be cached.

22. adjust hardware
1) add more memory to the machine;
2) increase the speed of the hard disk to reduce the I/O wait time;
Track-seeking time is the main factor that determines performance. the speed of moving the head is the slowest word by word. once the head is located, it is fast to read from the track;
3) re-allocate disk activities on different physical hard disk devices;
If possible, the busiest data inventory should be placed on different physical devices, which is different from using different partitions of the same physical device, because they will compete for the same physical resources (head)

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