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Technical Articles

Database

SQL

What does _ in sql mean?

In SQL, the uses of underscore (_) include: 1. As a placeholder in a query statement; 2. Used to escape special characters; 3. Identify SQL reserved words; 4. Create privileged aliases; 5. Replace columns with Concatenate into a string.

May 01, 2024 pm 10:18 PM

jion usage in mysql

The JOIN operation in MySQL is used to merge data from two or more tables. It joins tables with related column values by specifying conditions. Common types include: INNER JOIN: Returns rows with matching rows. LEFT JOIN: Returns all rows from the left table, even if there is no match in the right table. RIGHT JOIN: Returns all rows from the right table, even if there is no match in the left table. FULL JOIN: Returns all rows from both tables, even if there is no match. Benefits of JOIN include merging data, reducing redundancy, and improving query performance.

May 01, 2024 pm 10:18 PM

mysql

The role of use in mysql

MySQL's USE command is used to select the database to use in a session and apply subsequent queries and operations to the specified database.

May 01, 2024 pm 10:16 PM

mysql

What does using mean in sql

The USING keyword is used in SQL to join tables with common columns. Its main functions include: Joining tables: USING can join two or more tables to query or operate their data. Specify join conditions: USING allows you to specify join conditions that compare values in common columns to determine which rows to join.

May 01, 2024 pm 10:15 PM

How to delete a view in sql

In SQL, you can delete a view by following these steps: Connect to the database. Switch to the mode containing the view. Use the DROP VIEW <view name>; statement to delete a view.

May 01, 2024 pm 10:15 PM

What is the command to delete a view in sql

Use the DROP VIEW statement in SQL to delete a view. The syntax is as follows: DROP VIEW [IF EXISTS] view_name; where view_name is the name of the view to be deleted; IF EXISTS is optional. If the view does not exist, no error will be generated.

May 01, 2024 pm 10:12 PM

What command can be used in sql to modify the data in the table?

The command to modify table data in SQL is UPDATE, and the syntax is UPDATE table_name SET column_name = new_value WHERE condition. Among them, table_name is the table name, column_name is the column name, new_value is the new value, and condition is the filter condition, used to specify which rows to update. Example: UPDATE customers SET age = 30 WHERE name = "John Doe", set the name column value in the customers table to "John D

May 01, 2024 pm 10:12 PM

The command to modify data in the table in sql is

The most commonly used command in SQL to modify data in a table is UPDATE, which is used to update data in specified columns of rows that meet conditions. The command syntax is as follows: UPDATE table_name SET column_name = new_value WHERE condition;. For example: UPDATE customers SET address = '123 Main Street' WHERE name = 'John' AND age > 30; will list "name" in the table "customers" as "John" and "age"

May 01, 2024 pm 10:12 PM

What command is used to create a view in sql

In SQL, use the CREATE VIEW statement to create a view. The syntax is: Named view: The view name is followed by AS to define a subquery: The AS keyword is followed by specifying the subquery that is the basis of the view.

May 01, 2024 pm 10:09 PM

The command to modify the table structure in sql is

The command to modify the table structure in SQL is ALTER TABLE, which supports the following operations: Use ADD COLUMN to add a new column, use DROP COLUMN to delete an existing column, use ALTER COLUMN to modify the properties of an existing column, use ADD FOREIGN KEY to add a foreign key constraint, and use DROP FOREIGN KEY to delete a foreign column. Key constraints use RENAME TABLE to modify the table name

May 01, 2024 pm 10:09 PM

How to modify data in sql

There are three main ways to modify data in SQL: UPDATE statement: Modifies existing data, specifying the columns and conditions to be updated. INSERT statement: Inserts a new row, specifying the columns and values to be inserted. DELETE statement: deletes rows with specified conditions.

May 01, 2024 pm 10:06 PM

Statements to modify data in sql

There are three statements that modify data in SQL: update (UPDATE), insert (INSERT) and delete (DELETE). The UPDATE statement is used to modify the value of an existing row, the INSERT statement is used to insert new rows, and the DELETE statement is used to delete rows.

May 01, 2024 pm 10:06 PM

What are the commands to modify data in sql

The commands to modify data in SQL mainly include the following categories: UPDATE command: update the specified field value in the row that meets the conditions. DELETE command: delete rows that meet the conditions. INSERT command: Inserts new rows into the table. MERGE command: Merge data into a table, supports updates and inserts. TRUNCATE command: Efficiently delete all rows in the table.

May 01, 2024 pm 10:03 PM

How to use sumif in sql

The SUMIF function in SQL is used to sum numeric values under specific conditions. The syntax is: specify the condition and data range to be applied. Use the SUMIF function in SQL queries, noting conditions, ranges, and summing ranges. Run a query to get the sum of the numeric values of cells that meet a condition.

May 01, 2024 pm 10:03 PM