Data modeling using ER

Subqueries and scripts & batches1Definition of a subqueryNested subqueriesCorrelated subqueries The ISNULL functionDerived tablesThe EXISTS operatorMixing data types: CAST & CONVERTPerformance considerationsWriting scripts Retrieving identity values@@ROWCOUNTBatches Control-of-flow statements

Steen Jensen, autumn 2013

Definition of a subqueryA subquery is a query nested inside another query

Generally used for one of a few needs:To break a query into a series of logical stepsTp provide a listing to be the target of a WHERE clauseTo provide a lookup driven by each individual record in a parent query

Most subqueries (but not all) can also be written as a join2Nested subqueriesA nested subquery is a query, which only goes in one direction returning either a single value for use in the outer query, or a full list of values to bused with the IN operator

See examples page 215 218bot3Correlated subqueriesA correlated subquery works in two directions

In a correlated subquery the inner query runs on information provided by the outer query, and vice versa

See examples page 218bot 223bot4The ISNULL functionThe ISNULL function accepts a variable or expression and tests it for a NULL value

If the value is NULL, the function returns some other specified value

If the value is not NULL, the original value is returned

See examples page 223bot 225bot

5Derived tablesA derived table is made up of the columns and rows of a result set from a query

To create a derived table, you must do two things:Enclose the query that generates the result set in parenthesesAlias the results of the query, so the alias can be referenced as a table

See examples page 215bot 227bot

6Parts, which can be skipped7Using common table expressions (CTEs): page 228top 232top

The EXISTS operatorWhen you use EXISTS, you dont return data but a simple TRUE/FALSE

See examples page 232 234bot

8Mixing data types: CAST & CONVERTBoth CAST and CONVERT perform data type conversions

You would use CAST and CONVERT in situations, when SQL Server would not implicitly make the conversions for you

See examples page 236top 239top

9Performance considerations10

Exercise in subqueriesExperiment running & changing the different subqueries at SQLZOO: http://sqlzoo.net/wiki/SELECT_.._SELECT

11Writing scriptsA script isnt a script until you store it in a file, where it can be pulled up and reused

Scripts are usually treated as a unit

Scripts can use both system functions and local variables

You can declare one variable (a scalar variable) at a time or several

The are three ways to set the value of a variable:Initialize it in the DECLARE statementUse a SELECT statementUse a SET statement

See examples page 384 388bot

12Common system functions13

Retrieving identity valuesBy using the system function called SCOPE_IDENTITY() the value of the last inserted key can be obtained

See examples page 391 394mid

14Parts, which can be skipped15Generating sequences: page 395 398bot

Running from the command prompt - SQLCMD: page 405top 409bot

Dynamic SQL using the EXEC command: page 409bot 415bot

@@ROWCOUNTBy using the system function called @@ROWCOUNT the number of affected rows can be obtained

See examples page 398bot 399mid

16Grouping statements into batchesA batch is a grouping of SQL statements into one logical unit

If a statement fails at parsetime, nothing runsIf a statement fails at runtime, all statements until the erroneous statement have already been run

To separate a script into multiple batches, you make use of the GO statement

The GO statement:Must be on a separate lineCauses all statements prior to the GO statement to be sent independently to the serverIs not a SQL statement

See examples page 400mid 401top + 404 405top

17When to use batchesBatches are used, when something has to happen either before or separately from everything else in a script

The following commands require their own batch:CREATE DEFAULTCREATE PROCEDURECREATE RULECREATE TRIGGERCREATE VIEW

Batches can be used to establish precedence

18Control-of-flow statementsControl-of-flow statements makes it possible to use programming language constructs in SQL

The control-of-flow statements include:IF...ELSEGOTOWHILEWAITFORTRY/CATCH

See examples page 417-418 + 419bot 420top + 421bot 423top + 425 426mid + 427mid + 429bot 430mid

19Exercise in scriptMake ex.1 on page 432 (chapter 11) in the book without looking at the answer

Then check your answer up against the indicative answer on page 790

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