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Technical Articles

Database

What does sql constraint mean?

SQL constraints are rules that limit data in tables to ensure data consistency, accuracy and integrity. They are divided into five categories: Primary key constraints: Ensure row unique identification. Foreign key constraints: Associate the table to ensure that the values in the referenced table exist in the referenced table. Unique constraints: Ensure that a specific column or group column contains unique values. Non-empty constraint: prevents columns from being empty. Check constraints: Verify that the data complies with custom rules.

Apr 09, 2025 pm 05:45 PM

What does sql wildcard mean?

SQL wildcards allow you to use special characters (%), (_), ([]) and ([^]) to match a wide range of values in a SELECT statement, thus writing more general queries. They are used to match any character, a single character, a sequence of characters within square brackets, and a sequence of characters outside square brackets, respectively. Use wildcards with caution, as different DBMS regulations differ, abuse may cause query performance problems.

Apr 09, 2025 pm 05:42 PM

What does sql descending and ascending order mean?

In SQL, DESC and ASC keywords are used to sort the query results in descending and ascending order respectively, the descending order is arranged from large to small, and the ascending order is arranged from small to large.

Apr 09, 2025 pm 05:39 PM

排列

What does unique mean in sql

In SQL, UNIQUE constraints ensure that values in one or more columns are unique and prevent duplication. To create a UNIQUE constraint, use the ALTER TABLE statement and specify the column to be constrained. The benefits of UNIQUE constraints include data integrity, quick lookup, and preventing data duplication. It should be noted that it does not apply to NULL values.

Apr 09, 2025 pm 05:36 PM

What does exec mean in sql

The EXEC command in SQL is used to execute stored procedures or dynamic SQL statements. It automates tasks, generates queries dynamically, and enhances procedurality by executing precompiled stored procedures, or generates and executes SQL queries at runtime.

Apr 09, 2025 pm 05:33 PM

What does sql injection or1=1 mean?

The "1=1" expression in SQL injection is always true, allowing an attacker to bypass query conditions, access or modify database data, including stealing sensitive information and modifying database records. To prevent SQL injection, user input needs to be validated and filtered, parameterized queries or prepared statements are used, and input validation functions are used to check the format and content of the input.

Apr 09, 2025 pm 05:30 PM

敏感数据防止sql注入

What does sql express mean

Microsoft SQL Express is a free and lightweight database management system for small businesses, developers and individual users. Features include: Free to use and distribute compact and lightweight and easy to install and use for small workloads compatible with full version of SQL Server

Apr 09, 2025 pm 05:27 PM

What does sql dbo mean

dbo is the database owner in SQL Server and has the highest permissions to the database, including creating and modifying database objects, granting and revoking permissions, and backing up and restoring the database. It is generally not recommended to use dbo for daily database operations, but to create new users with limited permissions for specific tasks. However, dbo may be required when installing or upgrading SQL Server, performing complex database operations, or granting/revoking permissions.

Apr 09, 2025 pm 05:24 PM

What does the SQL primary key and index mean?

Primary keys and indexes are key concepts in SQL for optimizing database management and performance. Primary keys force data uniqueness, while indexes improve query performance. The primary key value must be unique, and each table can have up to one primary key; the index values can be repeated, and each table can have multiple indexes. Primary keys and indexes work together to ensure data integrity, prevent duplicate recordings and improve query efficiency.

Apr 09, 2025 pm 05:21 PM

What does sql timestamp mean

SQL TIMESTAMP is a data type that stores timestamps with time zone offsets. It stores the number of seconds since the Unix time origin, with an optional additional microsecond value. Its advantages include high precision, time zone awareness and automatic time zone conversion.

Apr 09, 2025 pm 05:18 PM

What does sql varchar mean?

VARCHAR is a variable-length string data type used to store strings with unfixed lengths to improve space efficiency. VARCHAR allows variable-length strings, which are more efficient in space than CHAR, but may not perform as well as CHAR. When using VARCHAR, you should consider the maximum length of the field, avoid excessively long fields, and index the fields appropriately to improve query efficiency.

Apr 09, 2025 pm 05:15 PM

What does prevent SQL injection mean

Preventing SQL injection attacks is critical and follows: Input validation: Filter illegal characters and exception syntax. Parameterized query: Pass user input as parameters to prevent malicious code execution. Restrict user permissions: Grant the minimum necessary permissions. Data encoding: Prevent malicious code from being interpreted as SQL statements. Using a security framework: Follow best practices like OWASP Top 10. Deploy a database firewall: Filter and block malicious requests. Regular updates: patch security vulnerabilities. Security training: Increase awareness among developers and administrators. Continuous monitoring: Identify exceptional patterns and unauthorized access.

Apr 09, 2025 pm 05:12 PM

防止sql注入

What does sql server mean

SQL Server is Microsoft's relational database management system for storing, managing, and retrieving data. Its core functions include: data storage and management. Data query (via Transact-SQL language). Transaction processing. Security and permissions. High availability and fault tolerance.

Apr 09, 2025 pm 05:09 PM

What does sql where mean

The WHERE clause is an optional part of the filtering result in a SQL query. It uses a Boolean expression to limit the number of rows returned, filter data, and retrieve specific information. These expressions can use boolean operators, comparison operators, and more advanced features such as subqueries, IN/NOT IN, LIKE, and BETWEEN.

Apr 09, 2025 pm 05:06 PM