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Technical Articles

Database

SQL

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

db2 modify field length

You can modify the field length by using the SQL statement "alter table table name alter field name set data type field type (length);" in db2.

Sep 03, 2019 am 11:50 AM

db2

The specified connection identifier cannot be resolved

The solution to the failure to parse the specified connection identifier: first open plsql, click Cancel to enter the main interface; then select the toolbar tools-->preferences; then configure the Oracle information; finally click OK, close plsql, and restart.

Sep 02, 2019 am 11:22 AM

sql

SQL performance optimization

SQL performance optimization in databases has always been a very important issue. Today I will summarize some related issues for you.

Aug 19, 2019 am 10:30 AM

性能优化

Detailed introduction to sql injection

SQL Injection: By inserting SQL commands into Web form submissions or entering query strings for domain names or page requests, it ultimately deceives the server into executing malicious SQL commands. This article will introduce you to the relevant knowledge about SQL injection in detail.

Aug 06, 2019 pm 05:06 PM

sql

How to write sql is not equal to

There are two ways to express inequality in SQL, one is "<>" (without quotation marks), the other is "!=" (without quotation marks), the usage is the same. That is where a <> 100; or where a != 100;

Aug 03, 2019 pm 02:14 PM

sql