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Database

SQL

How to use sql datetime

The DATETIME data type is used to store high-precision date and time information, ranging from 0001-01-01 00:00:00 to 9999-12-31 23:59:59.99999999, and the syntax is DATETIME(precision), where precision specifies the accuracy after the decimal point (0-7), and the default is 3. It supports sorting, calculation, and time zone conversion functions, but needs to be aware of potential issues when converting precision, range and time zones.

Apr 09, 2025 pm 06:09 PM

数据丢失

How to use the sql round field

The SQL ROUND() function rounds the number to the specified number of digits. It has two uses: 1. num_digits>0: rounded to decimal places; 2. num_digits<0: rounded to integer places.

Apr 09, 2025 pm 06:06 PM

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What does sql foreign key constraint mean?

Foreign key constraints specify that there must be a reference relationship between tables to ensure data integrity, consistency, and reference integrity. Specific functions include: data integrity: foreign key values must exist in the main table to prevent the insertion or update of illegal data. Data consistency: When the main table data changes, foreign key constraints automatically update or delete related data to keep them synchronized. Data reference: Establish relationships between tables, maintain reference integrity, and facilitate tracking and obtaining related data.

Apr 09, 2025 pm 06:03 PM

What does sql pagination mean?

SQL paging is a technology that searches large data sets in segments to improve performance and user experience. Use the LIMIT clause to specify the number of records to be skipped and the number of records to be returned (limit), for example: SELECT * FROM table LIMIT 10 OFFSET 20; advantages include improved performance, enhanced user experience, memory savings, and simplified data processing.

Apr 09, 2025 pm 06:00 PM

What does sql foreign key mean

SQL foreign keys are defined as relationship constraints between tables, ensuring data consistency by associating foreign keys and primary keys. The functions include: maintaining data consistency, preventing deletion of dependent data, and mandatory reference integrity. The syntax is CREATE TABLE child_table (child_id INT PRIMARY KEY, parent_id INT, FOREIGN KEY (parent_id) REFERENCES parent_table (parent_id)). The advantages are guaranteed data integrity, manage deletion dependencies and simplify queries, and the disadvantages are that they may reduce performance and increase inter-table relationship complexity.

Apr 09, 2025 pm 05:57 PM

What does sql declare mean

In SQL, the DECLARE statement is used to declare variables or cursors, allowing data to be stored and used during program execution. Variables are used to store individual values, while cursors are used to traverse the result set. The DECLARE statement can also be used to store intermediate results, traverse the result set, and pass values to stored procedures or functions.

Apr 09, 2025 pm 05:54 PM

What does sql rowcount mean

ROWCOUNT in SQL is a system variable that returns the number of rows affected by the previous Transact-SQL statement. It can be used to check the execution of the statement, count the number of rows of the result set, or control the loop.

Apr 09, 2025 pm 05:51 PM

What does sql script mean

SQL scripts are files containing SQL statements used to interact with databases and automate complex operations. Its structure usually includes declaration, query, update and control statements. SQL scripts are executed via the command line or DBMS, providing automation, portability, version control, and documentation benefits.

Apr 09, 2025 pm 05:48 PM

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