MySQL

REPLACE runs like INSERT. There is only one exception. If an old record in the table has the same value as a new record used for the primary key or a UNIQUE index, the old record is deleted before the new record is inserted.Note: unless the table

Unique query for a single table: distinctUnique query for multiple tables: groupWhen distinct queries multiple tables, left join is still valid and full join is invalid,When using mysql, you sometimes need to query records with unique fields.

Today, the teacher gave me a review of the pages in the database. In general, today is a good day, because I have not forgotten what I learned before. Now, go to the topic,First, let's talk about the top method.The top method is to remove the data

1. Syntax IntroductionThere are three tables a, B, and c. Now we need to query the values of several fields from Table B and Table c and insert them to the corresponding fields in table. In this case, you can use the following statement:Insert into

The basic process of mysqld_safe script execution:1. Search for basedir and ledir.2. Search for datadir and my. cnf.3. Check my. cnf. For details about the options, see the annotations in the attachment.4. parse the groups [mysqld] and [mysqld_safe]

Drop user deletes all user information, while delete only deletes the user table. Other information, such as the information in the db table, still exists.If you create a user with the minimum permission after the delete operation, the user will

The innodb Engine differs greatly from the MYISAM engine. In particular, its data storage format.For the innodb data structure, we must first solve two conceptual problems: the shared tablespace and the exclusive tablespace.What are shared and

The creation and merging of mysql tables is similar to that of partition tables. It is a feature of mysql that combines multiple myisam tables with the same structure into a virtual table. The surviving engine must be merge, the query from a merged

A temporary table exists when the connection is not disconnected. Once it is disconnected, it does not exist. The data and structure of the temporary table are in the memory. You can perform a test to create a temporary table, but it is not found in

I saw a buddy write a tutorial and record it. I am ready to lose weight at any time.1. Delete the mysql-test folder; 2. Delete the SQL-test folder;3. Delete the debug folder under the bin directory;4. Delete all files with the. pdb extension in the

1. view the mysql database sizeSELECT sum (DATA_LENGTH) + sum (INDEX_LENGTH)FROM information_schema.TABLES where TABLE_SCHEMA = 'database name ';The result is in bytes. Except 1024 for K, except 1048576 (= 1024*1024) for M. 2. view the last mysql

Yesterday, BOSS issued a command to allow me to quickly generate SQL statements using word macros, so that we do not need to input a piece of metadata when creating a table. To improve efficiency and save costs: After receiving the command, I

Mysql 5.5 is installed, and the character set is faulty. The previous mysql servers can be configured in the my. cnf configuration file. However, an error is reported even though this is not the case: At that time, no error logs were configured in

Set and enum data types are all string objects.Set can contain up to 64 elements, and any element in the set can be obtained. However, enum can only obtain one Wood Element in the set, which can contain a maximum of 65536 elements. That is to say,

Run the status command to view the Slow queries item. If the value is greater than 0 for a long time, the query execution time is too long.Copy codeThe Code is as follows:Reference content is as follows:Mysql> status;--------------Mysql Ver 11.18

MySQL mysqldump is used for export. The basic usage is as follows:Shell> mysqldump [OPTIONS] database [tables]If you do not specify any tables, the entire database will be exported.Run mysqldump -- help to obtain the option table supported by your

When using storm to process logs, we often encounter the efficiency problem of inserting mysql concurrently. I checked some information online and made some notes.Change the table engine to myisam,Modify the concurrent_insert = 2 of my. cnf. The

On the surface of the database table, there is an index and error prevention mechanism. However, a simple query takes a long time. Web applications may run well in the development environment, but are equally bad in the product environment. If you

1. select the most suitable field attributeMySQL can support access to large data volumes, but generally, the smaller the table in the database, the faster the query will be executed on it. Therefore, when creating a table, we can set the field

This article mainly describes the analysis and summary of time functions in SQLite and provides use cases. The examples given in this article are all tested.SQLite time/date function types:1. datetime (): Date and time of Generation2. date (): date