The keywords of query commands in sql are
The command keyword for querying data in SQL is SELECT, which is used to extract specific columns or records from a database table. The SELECT statement structure includes: SELECT [DISTINCT]
FROM [WHERE
][ORDER BY ][LIMIT ]
Keywords for query commands in SQL: SELECT
Keywords for querying data in SQL The words are SELECT. It is used to extract specific columns or records from database tables.
SELECT statement structure:
SELECT [DISTINCT] <要提取的列名> FROM <表名> [WHERE <过滤条件>] [ORDER BY <排序列>] [LIMIT <返回记录数>]Copy after loginSELECT statement elements:
- DISTINCT: Used to exclude duplicate records.
- Column name to extract: Specify the column to be obtained from the table.
- FROM: Specify the table to query.
- WHERE: Specify filter conditions to filter out records that meet specific conditions.
- ORDER BY: Specify the column by which the results will be sorted.
- LIMIT: Specify the number of records to return.
Example:
The following statement queries the names and email addresses of all customers from the
Customerstable:SELECT Name, Email FROM Customers;Copy after loginThe following statement queries the product names and prices with a price greater than 100 from the
Productstable:SELECT Name, Price FROM Products WHERE Price > 100;Copy after loginThe following statement queries all orders in the past 30 days from the
Orderstable , and sorted by date in descending order:SELECT * FROM Orders WHERE OrderDate >= DATE('now', '-30 days') ORDER BY OrderDate DESC;Copy after loginThe above is the detailed content of The keywords of query commands in sql are. For more information, please follow other related articles on the PHP Chinese website!
Statement of this WebsiteThe content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
What's New in Windows 11 KB5054979 & How to Fix Update Issues3 weeks ago By DDDHow to fix KB5055523 fails to install in Windows 11?2 weeks ago By DDDInZoi: How To Apply To School And University4 weeks ago By DDDHow to fix KB5055518 fails to install in Windows 10?2 weeks ago By DDDWhere to find the Site Office Key in Atomfall4 weeks ago By DDDHot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
See all articlesThe 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.
How to create tables using SQL statements in SQL Server: Open SQL Server Management Studio and connect to the database server. Select the database to create the table. Enter the CREATE TABLE statement to specify the table name, column name, data type, and constraints. Click the Execute button to create the table.
SQL IF statements are used to conditionally execute SQL statements, with the syntax as: IF (condition) THEN {statement} ELSE {statement} END IF;. The condition can be any valid SQL expression, and if the condition is true, execute the THEN clause; if the condition is false, execute the ELSE clause. IF statements can be nested, allowing for more complex conditional checks.
There are two ways to deduplicate using DISTINCT in SQL: SELECT DISTINCT: Only the unique values of the specified columns are preserved, and the original table order is maintained. GROUP BY: Keep the unique value of the grouping key and reorder the rows in the table.
Common SQL optimization methods include: Index optimization: Create appropriate index-accelerated queries. Query optimization: Use the correct query type, appropriate JOIN conditions, and subqueries instead of multi-table joins. Data structure optimization: Select the appropriate table structure, field type and try to avoid using NULL values. Query Cache: Enable query cache to store frequently executed query results. Connection pool optimization: Use connection pools to multiplex database connections. Transaction optimization: Avoid nested transactions, use appropriate isolation levels, and batch operations. Hardware optimization: Upgrade hardware and use SSD or NVMe storage. Database maintenance: run index maintenance tasks regularly, optimize statistics, and clean unused objects. Query
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.
The DECLARE statement in SQL is used to declare variables, that is, placeholders that store variable values. The syntax is: DECLARE <Variable name> <Data type> [DEFAULT <Default value>]; where <Variable name> is the variable name, <Data type> is its data type (such as VARCHAR or INTEGER), and [DEFAULT <Default value>] is an optional initial value. DECLARE statements can be used to store intermediates
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.
