SQL CREATE VIEW, REPLACE VIEW, DROP VIEW Statements. SQL CREATE VIEW Statement. In SQL, a view is a virtual table based on the result- set of an SQL statement. A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from one single table. CREATE VIEW Syntax. CREATE VIEW view_name ASSELECT column. FROM table_name. WHERE condition; Note: A view always shows up- to- date data! The database engine recreates the data, using the view's SQL statement, every time a user queries a view. SQL CREATE VIEW Examples. If you have the Northwind database you can see that it has several views installed by default. The view "Current Product List" lists all active products (products that are not. SQL CREATE VIEW Statement. In SQL, a view is a virtual table based on the result-set of an SQL statement. A view contains rows and columns, just like a real table.
Products" table. The view is created with the following SQL. CREATE VIEW [Current Product List] ASSELECT Product. ID, Product. Name. FROM Products. WHERE Discontinued = No; Then, we can query the view as follows. SELECT * FROM [Current Product List]; Another view in the Northwind sample database selects every product in the "Products" table. CREATE VIEW [Products Above Average Price] ASSELECT Product. Name, Unit. Price. FROM Products. WHERE Unit. Price > (SELECT AVG(Unit. Price) FROM Products); We can query the view above as follows. SELECT * FROM [Products Above Average Price]; Another view in the Northwind database calculates. Note that this view selects its data. Product Sales for 1. CREATE VIEW [Category Sales For 1. ASSELECT DISTINCT Category. Name, Sum(Product. Sales) AS Category. Sales. FROM [Product Sales for 1. GROUP BY Category. Name; We can query the view above as follows. SELECT * FROM [Category Sales For 1. We can also add a condition to the query. Let's see the total sale only for the category "Beverages". SELECT * FROM [Category Sales For 1. WHERE Category. Name = 'Beverages'; SQL Updating a View. You can update a view by using the following syntax: SQL CREATE OR REPLACE VIEW Syntax. CREATE OR REPLACE VIEW view_name ASSELECT column. FROM table_name. WHERE condition; Now we want to add the "Category" column to the "Current Product List" view. We will update the view with the following SQL. CREATE OR REPLACE VIEW [Current Product List] ASSELECT Product. ID, Product. Name, Category. FROM Products. WHERE Discontinued = No; SQL Dropping a View. You can delete a view with the DROP VIEW command. SQL DROP VIEW Syntax. DROP VIEW view_name. My. SQL UPDATE Statement - Updating Data In a Table. Summary: updating data is one of the most important tasks when you work with the database. In this tutorial, you will learn how to use the My. SQL UPDATE statement to update data in a table. Introduction to My. SQL UPDATE statement. We use the UPDATE statement to update existing data in a table. We can use the UPDATE statement to change column values of a single row, a group of rows, or all rows in a table. The following illustrates the syntax of the My. SQL UPDATE statement. UPDATE [LOW_PRIORITY] [IGNORE] table_name. UPDATE[LOW_PRIORITY][IGNORE]table_name. In the UPDATE statement: First, specify the table name that you want to update data after the UPDATE keyword. Second, the SET clause specifies which column that you want to modify and the new values. To update multiple columns, you use a list comma- separated assignments. You supply the value in each column’s assignment in the form of a literal value, an expression, or a subquery. Third, specify which rows to be updated using a condition in the WHERE clause. The WHERE clause is optional. If you omit the WHERE clause, the UPDATE statement will update all rows in the table. Notice that the WHERE clause is so important that you should not forget. Sometimes, you may want to change just one row; However, you may forget the WHERE clause and accidentally updates all the rows in the table. My. SQL supports two modifiers in the UPDATE statement. The LOW_PRIORITY modifier instructs the UPDATE statement to delay the update until there is no connection reading data from the table. The LOW_PRIORITY takes effect for the storage engines that use table- level locking only, for example, My. ISAM, MERGE, MEMORY. The IGNORE modifier enables the UPDATE statement to continue updating rows even if errors occurred. The rows that cause errors such as duplicate- key conflicts are not updated. My. SQL UPDATE examples. Let’s practice the UPDATE statement with some tables in the My. SQL sample database. My. SQL UPDATE a single column example. In this example, we are going to update the email of Mary Patterson to the new email mary. First, to make sure that we update the email successfully, we query Mary’s email from the employees table using the following SELECT statement. Number = 1. 05. 6; Second, we can update Mary’s email to the new email mary. UPDATE statement as the following query. UPDATE employees. Number = 1. 05. 6; email='mary. Because we just want to update one row, we use the WHERE clause to specify the row using the employee number 1. The SET clause sets the value of the email column to the new email. Third, we execute the SELECT statement again to verify the change. Number = 1. 05. 6; My. SQL UPDATE multiple columns. To update values in the multiple columns, you need to specify the assignments in the SET clause. For example, the following statement updates both last name and email columns of employee number 1. UPDATE employees. Hill'. email = 'mary. Number = 1. 05. 6; email='mary. Let’s check the changes. Number = 1. 05. 6; My. SQL UPDATE from SELECT statement example. You can supply the values for the SET clause from a SELECT statement that queries data from other tables. For example, in the customers table, some customers do not have any sale representative. The value of the column sale. Rep. Employee. Number is NULL as follows. Rep. Employee. Number. Rep. Employee. Number IS NULL; customername,sales. Rep. Employee. Number sales. Rep. Employee. Number. ISNULL; We can take a sale representative and update for those customers. To do this, we can select a random employee whose job title is Sales Rep from the employees table and update it for the employees table. This query selects a random employee from the employees table whose job title is the Sales Rep. Number. jobtitle = 'Sales Rep'. ORDER BY RAND(). LIMIT 1; To update the sales representative employee number column in the customers table, we put the query above in the SET clause of the UPDATE statement as follows. UPDATE customers. Rep. Employee. Number = (SELECT. Number. employees. Sales Rep'. LIMIT 1). Rep. Employee. Number IS NULL; sales. Rep. Employee. Number=(SELECT sales. Rep. Employee. Number. ISNULL; If you query data from the employees table, you will see that every customer has a sales representative. In other words, the following query returns no row. Rep. Employee. Number. Rep. Employee. Number IS NOT NULL; sales. Rep. Employee. Number. ISNOT NULL; In this tutorial, you have learned how to use My. SQL UPDATE statement to update data in a database table.
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