SQL Workshop

Subqueries

Doug Shook

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Subqueries A SELECT statement within a SELECT statement

Four ways to code one:– WHERE (*)– HAVING (*)– FROM– SELECT

Syntax is the same– Rare to see GROUP BY or HAVING

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SubqueriesA subquery in the WHERE clause SELECT InvoiceNumber, InvoiceDate, InvoiceTotal FROM Invoices WHERE InvoiceTotal > (SELECT AVG(InvoiceTotal) FROM Invoices) ORDER BY InvoiceTotal;

The value returned by the subquery 1879.7413

The result set

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Subqueries vs. Joins Advantages of subqueries– Aggregates can be passed to outer query– More intuitive than joins for ad-hoc relationships– Can be easier to code

Advantages of joins– Can include columns from both tables– More intuitive than subqueries for existing

relationships– Performance is generally better

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Subqueries vs. Joins What would this look like with a subquery instead?

A query that uses an inner join SELECT InvoiceNumber, InvoiceDate, InvoiceTotal FROM Invoices JOIN Vendors ON Invoices.VendorID = Vendors.VendorID WHERE VendorState = 'CA' ORDER BY InvoiceDate;

The result set

( 4 0 r o w s )

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Subqueries vs. Joins

Most queries can be written either way

The same query restated with a subquery SELECT InvoiceNumber, InvoiceDate, InvoiceTotal FROM Invoices WHERE VendorID IN (SELECT VendorID FROM Vendors WHERE VendorState = 'CA') ORDER BY InvoiceDate;

The same result set

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Subqueries and WHERE

Subquery must return a single column of values here– Why?

Could this be rewritten without the subquery?

The syntax of a WHERE clause that uses an IN phrase with a subquery

WHERE test_expression [NOT] IN (subquery)

A query that returns vendors without invoices SELECT VendorID, VendorName, VendorState FROM Vendors WHERE VendorID NOT IN (SELECT DISTINCT VendorID FROM Invoices);

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Subqueries and WHERE

NOT IN can typically be replaced with an outer join.

The query restated without a subquery SELECT Vendors.VendorID, VendorName, VendorState FROM Vendors LEFT JOIN Invoices ON Vendors.VendorID = Invoices.VendorID WHERE Invoices.VendorID IS NULL;

The same result set

(88 rows)

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Subqueries and WHERE

What is the subquery returning here?

The syntax of a WHERE clause that uses a comparison operator WHERE expression comparison_operator [SOME|ANY|ALL] (subquery)

A query with a subquery in the WHERE condition SELECT InvoiceNumber, InvoiceDate, InvoiceTotal, InvoiceTotal - PaymentTotal - CreditTotal AS BalanceDue FROM Invoices WHERE InvoiceTotal - PaymentTotal - CreditTotal > 0 AND InvoiceTotal - PaymentTotal - CreditTotal < (SELECT AVG(InvoiceTotal - PaymentTotal - CreditTotal) FROM Invoices WHERE InvoiceTotal - PaymentTotal - CreditTotal > 0) ORDER BY InvoiceTotal DESC;

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ALL Can be thought of as a sequence of ANDs:

Special cases:– If no rows are returned, ALL will be true– What if all rows are null?

How the ALL keyword works Condition Equivalent expression x > ALL (1, 2) x > 2 x < ALL (1, 2) x < 1 x = ALL (1, 2) (x = 1) AND (x = 2) x <> ALL (1, 2) (x <> 1) AND (x <> 2)

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ALLA query that uses the ALL keyword SELECT VendorName, InvoiceNumber, InvoiceTotal FROM Invoices JOIN Vendors ON Invoices.VendorID = Vendors.VendorID WHERE InvoiceTotal > ALL (SELECT InvoiceTotal FROM Invoices WHERE VendorID = 34) ORDER BY VendorName;

The result of the subquery

The result set

(25 rows)

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ANY and SOME Can be thought of as a sequence of Ors

Special cases– No rows returned?– All null values?

How the ANY and SOME keywords work Condition Equivalent expression x > ANY (1, 2) x > 1 x < ANY (1, 2) x < 2 x = ANY (1, 2) (x = 1) OR (x = 2) x <> ANY (1, 2) (x <> 1) OR (x <> 2)

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ANYA query that uses the ANY keyword SELECT VendorName, InvoiceNumber, InvoiceTotal FROM Vendors JOIN Invoices ON Vendors.VendorID = Invoices.VendorID WHERE InvoiceTotal < ANY (SELECT InvoiceTotal FROM Invoices WHERE VendorID = 115);

The result of the subquery

The result set

(17 rows)

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Correlated Subqueries Each subquery we’ve seen so far has been executed exactly once

Correlated subqueries execute once for each row in the outer query– Typically as part of a conditional statement

Potential conflicts– Same column in outer query and subquery?– Same table in outer query and subquery?

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Correlated SubqueriesA query that uses a correlated subquery SELECT VendorID, InvoiceNumber, InvoiceTotal FROM Invoices AS Inv_Main WHERE InvoiceTotal > (SELECT AVG(InvoiceTotal) FROM Invoices AS Inv_Sub WHERE Inv_Sub.VendorID = Inv_Main.VendorID) ORDER BY VendorID, InvoiceTotal;

The value returned by the subquery for vendor 95 28.5016

The result set

(36 rows)

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EXISTS Tests whether a subquery returns any rows– Does not return the rows, even if they exist– Most often used with correlated subquery

The syntax of a subquery with EXISTS WHERE [NOT] EXISTS (subquery)

A query that returns vendors without invoices SELECT VendorID, VendorName, VendorState FROM Vendors WHERE NOT EXISTS (SELECT * FROM Invoices WHERE Invoices.VendorID = Vendors.VendorID);

The result set

(88 rows)

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Subqueries with FROM This type of subquery returns a derived table– Must assign it to an alias– Can be used in the outer query like any other

table

Often used to provide additional summaries to a summary query

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Subqueries with FROM

A subquery coded in the FROM clause SELECT Invoices.VendorID, MAX(InvoiceDate) AS LatestInv, AVG(InvoiceTotal) AS AvgInvoice FROM Invoices JOIN (SELECT TOP 5 VendorID, AVG(InvoiceTotal) AS AvgInvoice FROM Invoices GROUP BY VendorID ORDER BY AvgInvoice DESC) AS TopVendor ON Invoices.VendorID = TopVendor.VendorID GROUP BY Invoices.VendorID ORDER BY LatestInv DESC;

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Subqueries with FROMThe derived table generated by the subquery

The result set

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Subqueries with SELECT Used in place of a column specifier– Must return a single value– Usually correlated– Can often be stated with a join instead• And should be!

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Subqueries with SELECT

How could we restate this with a join?

A correlated subquery in the SELECT clause SELECT DISTINCT VendorName, (SELECT MAX(InvoiceDate) FROM Invoices WHERE Invoices.VendorID = Vendors.VendorID) AS LatestInv FROM Vendors ORDER BY LatestInv DESC;

The result set

(122 rows)

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Subqueries with SELECTThe same query restated using a join SELECT VendorName, MAX(InvoiceDate) AS LatestInv FROM Vendors LEFT JOIN Invoices ON Vendors.VendorID = Invoices.VendorID GROUP BY VendorName ORDER BY LatestInv DESC;

The same result set

( 1 2 2 r o w s )

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Complex QueriesA complex query that uses three subqueries SELECT Summary1.VendorState, Summary1.VendorName, TopInState.SumOfInvoices FROM (SELECT V_Sub.VendorState, V_Sub.VendorName, SUM(I_Sub.InvoiceTotal) AS SumOfInvoices FROM Invoices AS I_Sub JOIN Vendors AS V_Sub ON I_Sub.VendorID = V_Sub.VendorID GROUP BY V_Sub.VendorState, V_Sub.VendorName) AS Summary1 A complex query (continued) JOIN (SELECT Summary2.VendorState, MAX(Summary2.SumOfInvoices) AS SumOfInvoices FROM (SELECT V_Sub.VendorState, V_Sub.VendorName, SUM(I_Sub.InvoiceTotal) AS SumOfInvoices FROM Invoices AS I_Sub JOIN Vendors AS V_Sub ON I_Sub.VendorID = V_Sub.VendorID GROUP BY V_Sub.VendorState, V_Sub.VendorName) AS Summary2 GROUP BY Summary2.VendorState) AS TopInState ON Summary1.VendorState = TopInState.VendorState AND Summary1.SumOfInvoices = TopInState.SumOfInvoices ORDER BY Summary1.VendorState;

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Complex Queries

Such queries are useful, but can we simplify this process?

The result set

( 1 0 r o w s )

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Complex Queries Procedure– State the problem to be solved– Use pseudocode to outline the query• Identify subqueries• Identify results from subqueries• Include aliases

– Use pseudocode for each subquery– Code the subqueries• Test!

– Combine and test final query

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Complex Queries

Pseudocode for the query SELECT Summary1.VendorState, Summary1.VendorName, TopInState.SumOfInvoices FROM (Derived table returning VendorState, VendorName, SumOfInvoices) AS Summary1 JOIN (Derived table returning VendorState, MAX(SumOfInvoices)) AS TopInState ON Summary1.VendorState = TopInState.VendorState AND Summary1.SumOfInvoices = TopInState.SumOfInvoices ORDER BY Summary1.VendorState;

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Complex QueriesThe code for the Summary1 and Summary2 subqueries SELECT V_Sub.VendorState, V_Sub.VendorName, SUM(I_Sub.InvoiceTotal) AS SumOfInvoices FROM Invoices AS I_Sub JOIN Vendors AS V_Sub ON I_Sub.VendorID = V_Sub.VendorID GROUP BY V_Sub.VendorState, V_Sub.VendorName;

The result of the Summary1 and Summary2 subqueries

( 3 4 r o w s )

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Complex QueriesPseudocode for the TopInState subquery SELECT Summary2.VendorState, MAX(Summary2.SumOfInvoices) FROM (Derived table returning VendorState, VendorName, SumOfInvoices) AS Summary2 GROUP BY Summary2.VendorState;

The result of the TopInState subquery

( 1 0 r o w s )

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Common Table Expressions Allows you to code an expression that defines a derived table– This table can be used with the query immediately

following the CTE

Most often used with SELECT

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Common Table ExpressionsTwo CTEs and a query that uses them WITH Summary AS ( SELECT VendorState, VendorName, SUM(InvoiceTotal) AS SumOfInvoices FROM Invoices JOIN Vendors ON Invoices.VendorID = Vendors.VendorID GROUP BY VendorState, VendorName ), TopInState AS ( SELECT VendorState, MAX(SumOfInvoices) AS SumOfInvoices FROM Summary GROUP BY VendorState ) SELECT Summary.VendorState, Summary.VendorName, TopInState.SumOfInvoices FROM Summary JOIN TopInState ON Summary.VendorState = TopInState.VendorState AND Summary.SumOfInvoices = TopInState.SumOfInvoices ORDER BY Summary.VendorState;

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Exercises Write a SELECT statement that returns the same result set as this SELECT statement. Substitute a subquery in a WHERE clause for the inner join.

SELECT DISTINCT VendorNameFROM Vendors JOIN InvoicesON Vendors.VendorID = Invoices.VendorIDORDER BY VendorName;

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Exercises Write a SELECT statement that answers this question: Which invoices have a PaymentTotal that's greater than the average PaymentTotal for all paid invoices? Return the InvoiceNumber and InvoiceTotal for each invoice.

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Exercises Write a SELECT statement that answers this question: which invoices have a PaymentTotal that's greater than the median PaymentTotal for all paid invoices? Return the InvoiceNumber and InvoiceTotal for each invoice.

Hint: Start with the solution to the previous problem then use the ALL keyword in the WHERE clause and code "TOP 50 PERCENT PaymentTotal" in the subquery.

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Exercises Write a SELECT statement that returns two columns from the GLAccounts table: AccountNo and AccountDescription. The result set should have one row for each account number that has never been used. Use a correlated subquery introduced with the NOT EXISTS operator. Sort the final result set by AccountNo.

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Exercises Write a SELECT statement that returns four columns: VendorName, InvoiceID, InvoiceSequence, and InvoiceLineItemAmount for each invoice that has more than one line item in the InvoiceLineItems table.

Hint: Use a subquery that tests for InvoiceSequence > 1.

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Exercises Write a SELECT statement that returns a single value that represents the sum of the largest unpaid invoices submitted by each vendor. Use a derived table that reutrns MAX(InvoiceTotal) grouped by VendorID, filtering for invoices with a balance due.

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Exercises Write a SELECT statement that returns the name, city, and state of each vendor that's located in a unique city and state. In other words, don't include vendors that have a city and state in common with another vendor.

Write a SELECT statement that returns four columns: VendorName, InvoiceNumber, InvoiceDate, and InvoiceTotal. Return one row per vendor, representing the vendor's invoice with the earliest date.