More examples of Nested Subqueries. IN / NOT IN – This operator takes the output of the inner query after the inner query gets executed which can be zero or more values and sends it to the outer query.

Parametrized queries have two main advantages: Security: It is a good way to avoid SQL Injection vulnerabilities; Performance: If you regularly invoke the same query just with different parameters a parametrized query might allow the database to cache your queries which is a considerable source of performance gain.

Jan 25, 2013 · Sample of the Recursive Level: DECLARE @VALUE_CODE AS VARCHAR(5); --SET @VALUE_CODE = 'A' -- Specify a level WITH ViewValue AS ( SELECT ValueCode , ValueDesc , PrecedingValueCode FROM ValuesTable WHERE PrecedingValueCode IS NULL UNION ALL SELECT A.ValueCode , A.ValueDesc , A.PrecedingValueCode FROM …

Sep 23, 2012 · The SQL WITH clause was introduced by Oracle in the Oracle 9i release 2 database. The SQL WITH clause allows you to give a sub-query block a name (a process also called sub-query refactoring), which can be referenced in several places within the main SQL query. The name assigned to the sub-query is treated as though it was an inline view or table.

The other thing you might want to do is profile your SQL Server execution stack and find out if all the queries are indeed the correct ones, e.g., if you are using just one query which returns both correct and incorrect results, then clearly the query being used is in question, but what about if your application is sending out different queries ...

May 23, 2017 · The impetus for this question is the SQL query discussed in this question about a complex join. After staring at the answer queries for a number of minutes I finally decided to step through the query using particular records to see what was going on. That was the only way I could think of to understand the query piece by piece.

Mar 23, 2013 · If it has to be "nested", this would be one way to get your job done: SELECT o.name AS country, o.headofstate FROM country o WHERE o.headofstate like 'A%' AND ( -- guaranteed to return a single value SELECT i.population FROM city i WHERE i.id = o.capital ) …

There are certain key points in writing queries but to me it looks like you already have an idea of those so optimizing in your case is much more about this than any general rules. After a few years of db development I did look at a few books specifically aimed at database optimization on the SQL Server and found very little useful info.

In order to practice queries the first thing you will need is data to run your queries on, hence the sample databases. After that just start reading about different sql statements and trying them out on your sample data.

If it's not, then it's safe to use in a dynamic SQL statement (and dynamic SQL is ultimately your only option here: even the user defined function has you doing that at some level). Oh, and one more thing--my choice of names and types for the lookup table is not an accident. The SQL Standard already has a table like this (well, a view anyway).