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I want to select the tests that run on IE7 and not run on IE8. I tried this but i get 0 and this is not true.
SELECT test_name FROM tests WHERE version='ie7' AND NOT EXISTS (SELECT test_name FROM tests where version='ie8');
SELECT test_name FROM tests WHERE version='ie7' AND test_name NOT In (SELECT test_name FROM tests where version='ie8');
SQL Exists vs. IN clause, It would refer to my_table in the external query only if t2 had no id column. If you had an EXISTS version, it wouldn't work as you expected. SELECT test_name FROM tests t1 WHERE version='ie7' AND NOT EXISTS (SELECT test_name FROM tests t2 where test_name = t1.test_name AND version='ie8'); (My Transact-SQL is a bit rusty, but I think this is how it is done.
You probably mean:
SELECT test_name FROM tests t1 WHERE version='ie7' AND NOT EXISTS (SELECT test_name FROM tests t2 where test_name = t1.test_name AND version='ie8');
(My Transact-SQL is a bit rusty, but I think this is how it is done. The important thing is that you were saying "I want all test_names with version 'ie7' and also that no row exists in the database at all with version 'ie8'" :) )
Best way to test if a row exists in a MySQL table, SELECT column1 FROM t1 WHERE EXISTS (TABLE t2);. The results are the same as when using SELECT * with no WHERE clause in the subquery. No they are not the same thing, as they will not return the same rowset in the most simplistic use case. The LEFT OUTER JOIN will return all rows from the left table, both where rows exist in the related table and where they does not. The WHERE NOT EXISTS() subquery will only return rows where the relationship is not met.
This is a more optimal form of the correct query:
SELECT tests.test_name FROM tests LEFT JOIN tests AS tests2 ON tests.test_name = tests2.test_name AND tests2.version = 'ie8' WHERE tests.version = 'ie7' AND tests2.version IS NULL
You see I've added a comparison check for the test_name, as without it, you are saying get all tests for ie7 only if there are no ie8 tests at all.
Subqueries are less efficient than left joins, and this IS NULL condition check will produce the same result, and will allow for quicker processing with a good index.
This website would not exist without the advertisements we display and your kind there is at least one record in the orders table with the same customer_id. I always default to NOT EXISTS.. The execution plans may be the same at the moment but if either column is altered in the future to allow NULLs the NOT IN version will need to do more work (even if no NULLs are actually present in the data) and the semantics of NOT IN if NULLs are present are unlikely to be the ones you want anyway.
You're missing a condition on the subquery to match rows to the outer SELECT e.g.:
SELECT t.test_name FROM tests t WHERE t.version='ie7' AND NOT EXISTS ( SELECT test_name FROM tests where test_name=t.test_name AND version='ie8');
EXISTS and NOT EXISTS are used with a subquery in WHERE clause to examine if the result The query engine then moves onto next row in the Customers table and repeat Step 1 to 4 This query uses INNER JOIN and returns the same If PAYMENT.TS does not exist for a certain value (e.g. PAYMENT.TS=4), then there is no value to join on and the left join does not return a row #4. I tried using NOT IN / NOT EXISTS as a condition, but this only treats the case where the right table has explicit NULLS and not the case where no value exists.
SELECT test_name FROM tests WHERE version = 'ie7' EXCEPT SELECT test_name FROM tests WHERE version = 'ie8';
Because sometimes we want to do incremental add, so I would like to add for the query to only inserts those are not exist in the same table. SELECT * from employees WHERE NOT EXISTS ( SELECT name FROM eotm_dyn WHERE eotm_dyn.employeeid = employees.employeeid ) assuming that the two tables are linked by a foreign key relationship. At this point you could use a variety of other options including a LEFT JOIN. The optimizer will typically handle them the same in most cases, however.
However, the query optimizer now treats EXISTS and IN the same way, uses a Merge Join (Right Anti Semi Join) between the two tables. When updating the same table from a subquery, is Where Exists required? Breadcrumb. Question and Answer. Thanks for the question, Chad.
How do you use exists instead of in in SQL? However, the query optimizer now treats EXISTS and IN the same way, whenever it can, so you’re unlikely to see any significant performance differences. Nevertheless, you need to be cautious when using the NOT IN operator if the subquery’s source data contains NULL values. If so, you should consider using a NOT EXISTS operator instead of NOT
engine will scan all records fetched from the inner query. SQL NOT EXISTS Syntax. The basic syntax of the NOT EXISTS in SQL Server can be written as: SELECT [Column Names] FROM [Source] WHERE NOT EXISTS (Write Subquery to Check) Columns: It allows us to choose the number of columns from the tables. It may be One or more. Source: One or more tables present in the Database.
- Not in should be avoided as much as possible.
- faster than 'not exists'
- @Turek - I have to disagree. As always, you'd need to check for your scenario but NOT EXISTS can noticeably outperform LEFT JOIN..IS NULL. e.g. explainextended.com/2009/09/15/…
- Its hard to know what the situation is, e.g. the same blog says it's better to do LEFT JOIN..IS NULL in MySQL explainextended.com/2009/09/18/… . I must admit I only gave the answer because I knew it was best in MySQL.