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having usage

The function of the having clause is to filter the groups that meet the conditions. The having clause is restricted to the columns and aggregate expressions defined in the SELECT statement. The aggregate value needs to be referenced by repeating the aggregate function expression in the HAVING clause.

Jun 29, 2019 am 09:58 AM

having

Best Practices for MongoDB Indexes

Most developers know that indexing is faster. But in the actual process, we often encounter some questions & difficulties: the fields we query will have various cases. Do all fields involved in the query need to be indexed? How to choose between compound index and single field? Should I add both or a single field for each? This article attempts to explain the basic knowledge of indexing and answer the above questions

Jun 24, 2019 pm 05:36 PM

MongoDB索引

What are the architectural characteristics of a database system

The architectural features of the database system are three-level schema structure and two-level mapping, namely schema, outer schema, inner schema, outer schema/schema mapping, and schema/inner schema mapping.

Jun 24, 2019 pm 04:29 PM

数据库系统的体系结构特征

What the database design process does not include

The database design process does not include algorithm design. The database design process mainly includes requirements analysis, conceptual structure design, logical structure analysis, database physical design, database implementation, database operation and maintenance stages.

Jun 20, 2019 pm 01:37 PM

数据库设计

What are the types and characteristics of databases?

Database types: Databases are usually divided into two categories, namely relational databases and non-relational databases. Features: Relational databases are easy to maintain, easy to use, and complex to operate. As a supplement to relational databases, non-relational databases can achieve high efficiency and high performance in specific scenarios and characteristic problems.

Jun 19, 2019 pm 01:28 PM

数据库种类及特点

Detailed explanation of sql stored procedure examples

A stored procedure is a set of SQL statements designed to accomplish specific functions. It is similar to a programming language and includes data types, process control, input and output, and its own function library. We can learn more about the stored procedure in detail through the instance table Student, such as a parameterless stored procedure: select all the information in the Student table.

Jun 15, 2019 pm 02:05 PM

sql存储过程

What are the nine commonly used statements in SQL

The nine commonly used statements in SQL include create database, drop database, sql server, backup database, drop table tabname, drop index idxname, drop view view

Jun 15, 2019 am 11:16 AM

sql

What is the command to modify the table structure in sql statement?

The command in the SQL statement to modify the table structure is ALTER TABLE. The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.

Jun 14, 2019 pm 04:28 PM

sql修改表结构

What is the sql modification statement?

The sql modification statement is an UPDATE statement. The UPDATE statement is used to update existing records in the table. Please pay attention to the WHERE clause in the SQL UPDATE statement! The WHERE clause specifies which record or records need to be updated. If you omit the WHERE clause, all records will be updated!

Jun 14, 2019 pm 03:40 PM

sql修改

What is the database add statement?

The database add statement is the INSERT INTO statement. The INSERT INTO statement is used to insert new records into the table. The INSERT INTO statement can be written in two forms: the first form does not need to specify the column name of the data to be inserted, and only needs to provide the inserted value; the second form requires Specify the column name and the value to be inserted.

Jun 14, 2019 am 11:51 AM

数据库添加

A brief discussion of database stored procedures

Database stored procedures: Stored procedures are methods, with similar method names, variables to be passed by the method, and return results, so stored procedures have stored procedure names, stored procedure parameters, and return values.

Jun 14, 2019 am 11:25 AM

数据库存储过程

Detailed explanation of database stored procedures

Stored procedures can be viewed as stored t-sql scripts in the database. Why use stored procedures? 1. Increase performance; 2. Enhance security; 3. Use non-database technology in transact-sql; 4. Programming mode - use external programming language calls.

Jun 14, 2019 am 11:12 AM

数据库存储过程

What are the two methods of database backup?

The two methods for database backup are: 1. Use mysqldump combined with the exec function for database backup; 2. Use the [php+mysql+header] function for database backup.

Jun 14, 2019 am 11:02 AM

数据库备份

What are the transaction isolation levels of the database?

In database operations, in order to effectively ensure the correctness of concurrently read data, the transaction isolation level is proposed. A general database includes four isolation levels: read uncommitted, read committed, repeatable read, and serialized; different isolation levels handle transactions differently.

Jun 12, 2019 pm 05:08 PM

数据库事务隔离级别事务