The view comment is visible when performing a DESC FULL command for the view.Means in particular that an invalid view can still be usable. Otherwise, an appropriate error message is thrown. If this is successful, the view is valid again. At the next read access, the system tries to automatically recompile the A view is set to INVALID if one of its underlying objects was changed (see also column STATUS in system.The view creation text is limited to 2.000.000 characters.If the columns are not directly referenced in the subquery but assembled in expressions, then you must specify aliases for these either within the subquery or from the entire view. The clause if not exists is used for the creation of tables and is very useful for avoiding the error table already exists, as it will not create a table if. If the column aliases are not specified, then the column names are derived from the subquery. By specifying the column aliases, you can define the column names of the view.Note that syntax or other errors will not be found until the view is first used. This can be very useful if you want to create many views which depend on each other and which otherwise must be created in a certain order. By specifying the FORCE option, you can create a view without compiling its text.Keep in mind that existing object privileges on the view are dropped when replacing a view. An already existing view can be replaced with the OR REPLACE option, without having to explicitly delete the view with the DROP VIEW statement. ( MySQL 8.0.29 and later :) CREATE PROCEDURE IF NOT EXISTS, CREATE FUNCTION IF NOT EXISTS, or CREATE TRIGGER IF NOT EXISTS, if successful, is written in its entirety to the binary log (including the IF NOT EXISTS clause), whether or not the statement raised a warning because the object (procedure, function, or trigger) already existed.If NOT EXISTS (subquery) returns no rows, the result is TRUE.If NOT EXISTS (subquery) returns at least 1 row, the result is FALSE.If EXISTS ( subquery) returns no rows, the result is FALSE.If EXISTS ( subquery) returns at least 1 row, the result is TRUE., so if the server crashes between DROP and CREATE, the table will have been dropped, but not recreated, and you're left with no table at all. You do not need to specify column names, because the query tests for the existence or nonexistence of records that meet the conditions specified in the subquery. When using MariaDB's CREATE OR REPLACE, be aware that it behaves like DROP TABLE IF EXISTS foo CREATE TABLE foo. The system doesn't care about the real values in those records it just wants to know if a row is returned.Īlternatively, a subquery’s select list that uses EXISTS might consist of the asterisk (*). If the query returns, for example, five records, it returns 5 ones. In the above fragment, SELECT 1 returns the value 1 for every record in the query. MySQL ALTER TABLE does not have the IF EXISTS option. A common coding convention is to write all EXISTS tests as follows: EXISTS (SELECT 1 WHERE. Since the result depends only on whether any records are returned, and not on the contents of those records, the output list of the subquery is normally uninteresting. The EXISTS condition is considered to be met if the subquery returns at least one row. That does not work for me, since I do NOT want to replace the view if it exists. EXISTS subqueries take the following form: expression EXISTS ( subquery ) 1 Is there an elegant canonical way to create a view in a MySQL database ONLY if it does not exist, but don't do anything if the view already does exist There is a similar question here, whose answer is to use the 'CREATE OR REPLACE VIEW' syntax. Alternatively, if you're looking to CREATE or ALTER the view, regardless of if it exists, you could use an expression to change the keyword's value: DECLARE SQL nvarchar (MAX) SET SQL CASE WHEN EXISTS (SELECT name FROM sys.views WHERE name N'Report888View') THEN N'ALTER' ELSE N'CREATE' END + N' VIEW dbo. For example, EXISTS returns true if the subquery returns any rows, and EXISTS returns true if the subquery returns no rows. To drop an index with the 'IF EXISTS' clause, you can use the following syntax: DROP INDEX IF EXISTS indexname ON tablename Please note that 'IF EXISTS' is only available in MySQL 5.1.0 or later. You cannot associate a trigger with a TEMPORARY table or a view. The trigger becomes associated with the table named tblname, which must refer to a permanent table. Use EXISTS to identify the existence of a relationship without regard for the quantity. A trigger is a named database object that is associated with a table, and that activates when a particular event occurs for the table. The EXISTS predicate is one of the most common predicates used to build conditions that use noncorrelated and correlated subqueries.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |