1 Definition of a subquery Nested subqueries Correlated subqueries The ISNULL function Derived tables The EXISTS operator Mixing data types: CAST & CONVERT Performance considerations Writing scripts Retrieving identity values Batches Control-of-flow statements Steen Jensen, autumn 2013

 A 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 steps Tp provide a listing to be the target of a WHERE clause To provide a lookup driven by each individual record in a parent query  Most subqueries (but not all) can also be written as a join 2

 A 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 – 218bot 3

 A 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 – 223bot 4

 The 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 5

 A 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 parentheses Alias the results of the query, so the alias can be referenced as a table  See examples page 215bot – 227bot 6

7  Using common table expressions (CTEs): page 228top – 232top

 When you use EXISTS, you don’t return data but a simple TRUE/FALSE  See examples page 232 – 234bot 8

 Both 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 9

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 Experiment running & changing the different subqueries at SQLZOO:

 A script isn’t 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 statement Use a SELECT statement Use a SET statement  See examples page 384 – 388bot 12

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 By using the system function called SCOPE_IDENTITY() the value of the last inserted key can be obtained  See examples page 391 – 394mid 14

15  Generating sequences: page 395 – 398bot  Running from the command prompt - SQLCMD: page 405top – 409bot  Dynamic SQL – using the EXEC command: page 409bot – 415bot

 By using the system function called the number of affected rows can be obtained  See examples page 398bot – 399mid 16

 A batch is a grouping of SQL statements into one logical unit  If a statement fails at parsetime, nothing runs  If 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 line Causes all statements prior to the GO statement to be sent independently to the server Is not a SQL statement  See examples page 400mid – 401top – 405top 17

 Batches 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 DEFAULT CREATE PROCEDURE CREATE RULE CREATE TRIGGER CREATE VIEW  Batches can be used to establish precedence 18

 Control-of-flow statements makes it possible to use programming language constructs in SQL  The control-of-flow statements include: IF...ELSE GOTO WHILE WAITFOR TRY/CATCH  See examples page bot – 420top + 421bot – 423top – 426mid + 427mid + 429bot – 430mid 19

 Make 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