•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
•@@ROWCOUNT
•Batches
•Control-of-flow statements
Steen Jensen, autumn 2013
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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
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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
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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
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The ISNULL function accepts a variable or expression and tests it for a
NULL value
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If the value is NULL, the function returns some other specified value
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If the value is not NULL, the original value is returned
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See examples page 223bot – 225bot
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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
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Using common table expressions (CTEs): page 228top – 232top
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When you use EXISTS, you don’t return data but a simple TRUE/FALSE
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See examples page 232 – 234bot
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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
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Experiment running & changing the different subqueries at SQLZOO:
http://sqlzoo.net/wiki/SELECT_.._SELECT
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A script isn’t a script until you store it in a file, where it can be pulled up
and reused
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Scripts are usually treated as a unit
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Scripts can use both system functions and local variables
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You can declare one variable (a scalar variable) at a time or several
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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
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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
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Generating sequences: page 395 – 398bot
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Running from the command prompt - SQLCMD: page 405top – 409bot
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Dynamic SQL – using the EXEC command: page 409bot – 415bot
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By using the system function called @@ROWCOUNT the number of
affected rows can be obtained
See examples page 398bot – 399mid
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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 + 404 – 405top
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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
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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 417-418 + 419bot – 420top + 421bot – 423top + 425 –
426mid + 427mid + 429bot – 430mid
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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 790
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