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Technical Articles

Database

SQL

Functions of sql language

The functions of SQL language: 1. Data definition function, which can define the three-level schema structure of the database, namely external schema, global schema and internal schema structure; 2. Data manipulation function, including data insertion, deletion and deletion of basic tables and views. Modification; 3. The data control function is mainly to control the user's access rights to ensure the security of the system.

Feb 06, 2020 pm 01:16 PM

sql 功能

sql statement to modify data

SQL uses the update statement to modify data. The syntax format is "update <table name> set field 1 = value where <condition>". When multiple values need to be modified, each value in the set clause can be separated by commas.

Feb 06, 2020 am 11:02 AM

sql 修改数据

SQL fuzzy query statement

The syntax of the sql fuzzy query statement is "SELECT field FROM table WHERE certain field Like condition", among which conditions, SQL provides four matching modes, namely: [%], [_], [[]], [^].

Feb 06, 2020 am 10:50 AM

sql 模糊查询

What is the execution order of sql statements?

Execution order: the from clause assembles data from different data sources; the where clause filters record rows based on specified conditions; the group by clause divides the data into multiple groups; uses aggregate functions for calculations; uses the having clause to filter Group by; evaluate all expressions; select set output, etc.

Jan 07, 2020 pm 03:15 PM

sql语句执行顺序

What is the usage of in in sql

The IN operator allows us to specify multiple values in the WHERE clause. Syntax: SELECT column_name(s), FROM table_name, WHERE column_name IN (value1, value2,...).

Jan 07, 2020 pm 03:02 PM

sql in 用法

What is the usage of sql max

The MAX() function returns the maximum value in a column. NULL values are not included in the calculation. Syntax: SELECT MAX(column_name) FROM table_name. MIN() and MAX() can also be used on text columns to get the highest or lowest value in alphabetical order.

Jan 07, 2020 pm 02:47 PM

sql max 用法

What is the usage of sql as

as can be used to rename column names or table names. Query all data in the students table and name the students table ss, for example: select * from students as ss. The query results are displayed in Chinese, for example: age as age.

Jan 07, 2020 pm 02:37 PM

sql as 用法

What is the sql command to delete the testdb database?

The command to delete the testdb database is: "DROP DATABASE testdb;". In SQL, when you need to delete a created database, you can use the DROP DATABASE or DROP SCHEMA statement.

Dec 18, 2019 pm 01:57 PM

sql 命令

How to optimize SQL queries? (detailed explanation)

Optimization methods: 1. When designing the database: design of database tables, fields, and storage engines; 2. Make good use of the functions provided by MySQL itself, such as indexes and optimization of statement writing; 3. MySQL cluster, sub-database, sub-table, read and write separation.

Nov 30, 2019 pm 05:54 PM

SQL查询

Quickly understand the basic principles of sql injection

This article will introduce you to the basic principles of SQL injection and its harm! If you are interested, please come and take a look! My SQL statement is so simple, it can't be injected? After taking 5 minutes to read this complete example, you should never dare to take chances again!

Nov 29, 2019 pm 02:41 PM

sql注入原理

Commonly used statements of SQL triggers (summary)

This article summarizes some commonly used SQL trigger statements for everyone, which has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone. For more related SQL knowledge, please visit the SQL column!

Nov 23, 2019 pm 05:25 PM

SQL 触发器

What should I do if ora-12154: The specified connection identifier cannot be resolved?

When this kind of problem occurs, the first thing we think of is to check whether there is any problem with the service OracleOraDb11g_home2TNSListener. Open the service window and see if the OracleOraDb11g_homeTNSListener service is running. If not, start it.

Nov 16, 2019 am 09:38 AM

标识符

What should I do if there is too much data in SQL query and memory overflows?

Solution to memory overflow in SQL query with too much data: Use paging query statement, because paging query only queries a small amount of data each time, so it will not take up too much memory, and when the amount of data is large, paging query will save some time.

Oct 28, 2019 am 09:53 AM

sql