8/8/2023 0 Comments Mysql create view with union![]() ![]() INSERT INTO v3(c) VALUES ( 30) Code language: SQL (Structured Query Language) ( sql )īecause the v3 view does not have a WITH CHECK OPTION, and the statement conforms with the definition of the v2 view. However, the following insert statement works. The insert statement fails even though the row seems to conform with the definition of the v3 view.īecause the view v3 is dependent on the v2 view, and the v2 view has the option WITH CASCADED CHECK OPTION. CHECK OPTION failed 'classicmodels.v3' Code language: SQL (Structured Query Language) ( sql ) MySQL issued the following error message: Error Code: 1369. VALUES ( 8) Code language: SQL (Structured Query Language) ( sql ) Insert a new row into the t1 table through the v3 view with value 8. WHERE c < 20 Code language: SQL (Structured Query Language) ( sql ) It fails the new row that does not conform with the definition of v2 view.Īfter that, create a new view named v3 based on v2. CHECK OPTION failed 'classicmodels.v2' Code language: SQL (Structured Query Language) ( sql ) ![]() Now, insert a row with value 5 into the t1 table through the v2 view. WITH CASCADED CHECK OPTION Code language: SQL (Structured Query Language) ( sql ) Then, create a view v2 based on the v1 view with WITH CASCADED CHECK OPTION clause. VALUES ( 5) Code language: SQL (Structured Query Language) ( sql ) CREATE OR REPLACE VIEW v1Ĭ > 10 Code language: SQL (Structured Query Language) ( sql )īecause we did not specify the WITH CHECK OPTION, the following statement works even though it does not conform with the definition of the v1 view. Next, create a view v1 based on the t1 table with the data in the c column greater than 10. ) Code language: SQL (Structured Query Language) ( sql ) MySQL WITH CASCADED CHECK OPTIONĬonsider the following example to understand the effect of the WITH CASCADED CHECK OPTION:įirst, create a table named t1 with one column c whose data type is an integer. ![]() To determine the scope of the check, MySQL provides two options: LOCAL and CASCADED. If you don’t specify the keyword explicitly in the WITH CHECK OPTION clause, MySQL uses CASCADED by default. When you create a view with the WITH CHECK OPTION clause and issue a DML statement against the view such as INSERT, UPDATE, and DELETE, MySQL checks to ensure that the rows that are being changed are conformable to the definition of the view.īecause a view can be created based on other views, MySQL also checks the rules in the dependent views for data consistency. Introduction to scope of check LOCAL & CASCADED If this is not the case, you can follow the WITH CHECK OPTION tutorial. Note that you should be familiar with the WITH CHECK OPTION clause before going forward with this tutorial. Besides mailing lists, other applications for unions include when combining archived data with active data or comparing data in different tables that share a similar structure.Summary: in this tutorial, you will learn the differences between LOCAL and CASCADED in WITH CHECK OPTION clause. This results in a single list of all staff and customers, with a shared data structure. Instead, we simply combine the result sets, knowing that duplicates may exist. Notice that we use UNION ALL instead of UNION to prevent MySQL from eliminating duplicate rows, which can be computationally expensive when we have a large result set. SELECT first_name, last_name, email_address FROM customers UNION ALL SELECT first_name, last_name, email_address FROM staff You want to combine the results of both tables into a single result set, to produce a mailing list. UNIONs are best used when we need to combine two queries that would otherwise be difficult to create in one query.įor example, let's say you have two tables, customers and staff, both with similar column names. Looking at the above query, you'll see that this could be achieved with a simple SELECT statement. Also, bear in mind that the number of columns in all SELECT statements must be the same, with the same data types, or else you'll get an error. Notice how I used the UNION keyword to combine the two results. This will output the result set of 1 and 2, as if they were in a single column. Specifically, it takes the values from one table or query, and instead of putting them side-by-side with another table or another query, it puts them together over-under. A UNION query is used to combine the results of two or more SELECT statements into a single result set.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |