Page 31-SQL Programming Tutorials: Learn SQL Online-php.cn

Article Tags

All
web3.0
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Backend Development
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Web Front-end
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Database
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Operation and Maintenance
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Development Tools
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
PHP Framework
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Common Problem
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Other
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Tech
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
CMS Tutorial
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Java
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
System Tutorial
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Computer Tutorials
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Hardware Tutorial
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Mobile Tutorial
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Software Tutorial
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache
Mobile Game Tutorial
- Mysql Tutorial
- navicat
- SQL
- Redis
- phpMyAdmin
- Oracle
- MongoDB
- NoSQL database
- Memcached
- cloudera
- memcache

Home

Technical Articles

Database

SQL

Usage of like and in in sql

LIKE in SQL is used to find strings containing specific patterns. It supports wildcards, but is not as efficient as the IN operator. IN is used to find fields that match a specified list of values. It is faster and supports index optimization. It is recommended to use IN when looking for specific values, use LIKE when looking for similar strings, and use IN in preference to optimize performance.

May 02, 2024 am 03:39 AM

What does decode mean in sql

The DECODE function in SQL is a conversion function that converts an input expression into a specified value based on given conditions. The syntax is as follows: DECODE(expression, value1, result1, value2, result2, ..., default_result) The DECODE function checks the conditions one by one, finds a match and returns the corresponding result, otherwise it returns the default result. Equivalent to the CASE WHEN statement, but more concise and easier to read.

May 02, 2024 am 03:36 AM

What does sc in sql mean?

SC stands for SELECT COUNT in SQL, an aggregate function used to count the number of records whether or not a condition is met. SC syntax: SELECT COUNT(*) AS record_count FROM table_name WHERE condition, where COUNT(*) counts the number of all records, table_name is the table name, and condition is an optional condition (used to count the number of records that meet the condition).

May 02, 2024 am 03:33 AM

聚合函数

Is desc in ascending or descending order in sql?

The DESC keyword in SQL is used to specify that query results are sorted in descending order, that is, from maximum value to minimum value. It first retrieves the data, then sorts it by the value of the specified column from large to small, and then returns the sorted results.

May 02, 2024 am 03:30 AM

聚合函数排列

The role of foreign keys in sql

Foreign keys maintain data integrity and consistency in SQL, establishing a relationship between two tables by referencing the primary key of another table. They enforce referential integrity, ensuring that referencing records exist in referenced tables, and trigger cascading operations that automatically update related tables when records are modified or deleted. Foreign keys enhance data integrity and consistency, simplify data maintenance, and optimize query performance.

May 02, 2024 am 03:27 AM

What does the percent sign mean in sql

The percent sign (%) in SQL is a wildcard character that matches any number of characters. It can be used in: LIKE clause: compares data and patterns and can indicate matching any number of characters. WHERE clause: Filter data rows and select rows that meet the conditions. It can match any number of characters. Other uses include comment lines and placeholders in prepared statements.

May 02, 2024 am 03:24 AM

What is the default length of varchar in sql

The default length of the VARCHAR data type in SQL varies by DBMS, typically: MySQL: 255 characters PostgreSQL: 255 characters Microsoft SQL Server: 1 character (must be specified explicitly) Oracle: 2000 characters (if no length is specified) Factors that affect the default length include DBMS version, column constraints, and database design. You can determine the default VARCHAR length for a specific database by querying the information_schema.columns table. The best practice is to choose the appropriate VARCHAR length based on actual data needs to avoid excessive or inappropriate

May 02, 2024 am 03:18 AM

mysql oracle

How to use group by and order by together in sql

Grouped data can be sorted using GROUP BY and ORDER BY: 1. GROUP BY groups data; 2. ORDER BY sorts each group of data.

May 02, 2024 am 03:09 AM

聚合函数

How to write alphabetically in sql

In SQL, use the ORDER BY clause to sort data alphabetically. Sorting in ascending order uses ASC, while sorting in descending order uses DESC. When sorting by multiple columns, separate the column names with commas, for example: ORDER BY name ASC, age DESC.

May 02, 2024 am 03:03 AM

What does age mean in sql

The AGE function calculates the age difference between a specified date and another date. The syntax is: AGE(date_column, date_to_compare, [age_unit]). Parameters include the date column for which the age difference is to be calculated, the date used for comparison, and an optional age unit (default is years). Please note when using this function that a negative value means that date_to_compare occurs before date_column, and the AGE function only applies to date type data.

May 02, 2024 am 03:00 AM

The difference between union and union all in sql

UNION only returns unique rows, while UNION ALL contains duplicate rows; UNION deduplicates rows before merging, but UNION ALL does not deduplicate rows.

May 02, 2024 am 02:57 AM

What does order by mean in sql

The ORDER BY clause in SQL is used to arrange query results in a specific order. It uses the following syntax: ORDER BY column_name [ASC | DESC] [, ...], where ASC means ascending order and DESC means descending order. This clause can also sort by multiple columns, with priority determined by the order of the columns. By default, NULL values are ranked lowest, but this behavior can be modified with the NULLS FIRST or NULLS LAST clause.

May 02, 2024 am 02:48 AM

排列

What is the function of order by in sql

The ORDER BY clause is used to sort the rows in SQL query results by a specified column or expression to organize the results for analysis and reporting. Specific functions include: sorting rows in ascending or descending order, specifying the sort order, processing equal values, and supporting compound sorting.

May 02, 2024 am 02:39 AM

The role of where in sql

The WHERE clause is used to filter data by specifying conditions and retrieve only rows that meet the conditions. Detailed description: 1. Specify conditions and assign true values to rows that meet the conditions; 2. Exclude other rows and the result only contains rows that meet the conditions; 3. For example, all customers from a specific state can be retrieved; 4. There are also sub- Other functions such as queries, aggregate functions, and joins.

May 02, 2024 am 02:36 AM

聚合函数