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Technical Articles

Database

SQL

Subqueries are generally divided into several types

Subqueries are generally divided into three types: 1. where type subquery, which uses the inner query results as the comparison conditions of the outer query; 2. from type subquery, which uses the inner query results as a temporary table for the outer sql to be used again Query; 3. exists type subquery, get the results of the outer sql to the inner sql for testing.

Jun 18, 2020 pm 04:17 PM

子查询

Detailed explanation of database update usage

The database update statement is [UPDATE table name SET column name = new value WHERE column name = certain value]. Its function is to update the original data in the table. When used alone, use the where matching field.

Jun 18, 2020 pm 03:42 PM

数据库

What is the meaning of GROUP BY in SQL query?

In SQL queries, the meaning of GROUP BY is to group data according to specified rules. Its function is to group the query results according to the value of a certain column or multiple columns. Those with equal values are a group. The GROUP BY statement is generally used with the aggregate function. Use together.

Jun 17, 2020 pm 04:50 PM

sql

What is the statement to add fields in sql

The standard SQL statement for adding fields to a data table is [alter table table name add(field field type) [default 'Enter default value'] [null/not null] ;].

Jun 17, 2020 pm 04:43 PM

sql

Redis persistence configuration (picture and text introduction)

Redis is an open source, highly available non-relational database (NoSQL, not just a database) key-value database written in C language. Different from traditional databases, redis data is stored in memory, so the read and write performance is not generally high and can reach 100,000 operations per second.

Jun 17, 2020 pm 04:40 PM

redis

What is the sql command to delete a table?

The SQL command to delete a table is [drop table (name of the table to be deleted)], where drop is to delete the entire table to release space, and delete is to delete the contents of the table.

Jun 17, 2020 pm 04:35 PM

SQL

What is the command to create a data table in sql

The command to create a data table in SQL is: [CREATE TABLE table_name (column_name column_type);]. Columns between data tables should be separated by commas in English. SQL statements in data tables are not case-sensitive.

Jun 17, 2020 pm 01:43 PM

sql 数据表命令

What are the common data types in databases?

Commonly used data types in databases are: 1. Integer data type; 2. Floating point data type, used to store decimals; 3. Binary data type; 4. Logical data type; 5. Character data type, used to store various letters , digital symbols, special symbols.

Jun 16, 2020 pm 05:22 PM

数据库

Detailed explanation of sql isnull usage

In SQL, you can use [isnull] to replace the specified replacement value with NULL. This function is used to replace a NULL value. Its syntax is [ISNULL(check_expression, replacement_value)], and its parameter 1 checks whether it is NULL.

Jun 16, 2020 pm 04:54 PM

sql

What is an inner join?

Inner joins, the most common equivalent joins, are the earliest joins and are also called ordinary joins or natural joins. The effect is to delete all rows from the result table that do not match rows in other joined tables, so inner joins may lose information.

Jun 16, 2020 pm 04:42 PM

内连接

In sql language, what is the command to delete a table?

In SQL language, the command to delete a table is [drop], and the commands to insert, delete and query data in the table are [INSERT], [UPDATE] and [SELECT] respectively.

Jun 16, 2020 pm 03:09 PM

sql

What should I do if it prompts that restarting the computer fails when installing Server SQL2008?

The solution to the problem of failure to restart the computer when installing Server SQL2008 is: 1. Open the Registry Editor; 2. Right-click the folder [MSSQLServer] and select [Delete]; 3. Delete the pendingFielRenameOperations option.

Jun 16, 2020 am 11:35 AM

Server SQL2008

What are the basic database operation statements?

The basic operation statements of the database are: 1. To query the database, the statement is [SHOW DATABASES]; 2. To query the structure of the database, the statement is [CREATEDATABASE score]; 3. To create a database, the statement is [DATABASE score DEFAULT CHARSET].

Jun 15, 2020 pm 02:05 PM

数据库基本操作语句

What is the difference between hql and sql

The difference between hql and sql is: 1. The query is different, sql is oriented to database table query, hql is oriented to object query; 2. The judgment conditions are different, the class name followed by from in hql and the class object where are used as conditions for the object's attributes, sql What is followed by from is the table where and then the fields in the table are used to perform conditional queries.

Jun 15, 2020 pm 01:19 PM

sql