Joe Celko s SQL for Smarties - Advanced SQL Programming P45 pdf

CREATE TABLE TeamAssignments player_id INTEGER NOT NULL REFERENCES Playersplayer_id ON DELETE CASCADE ON UPDATE CASCADE, team_id CHAR5 NOT NULL REFERENCES Teamsteam_id ON DELETE CASCA

Trang 1

412 CHAPTER 19: PARTITIONING DATA IN QUERIES

WHERE S1.sup < S2.sup different suppliers AND S1.part = S2.part same parts

GROUP BY S1.sup, S2.sup HAVING COUNT(*) = (SELECT COUNT (*) same count of parts FROM SupParts AS S3

WHERE S3.sup = S1.sup) AND COUNT(*) = (SELECT COUNT (*)

FROM SupParts AS S4 WHERE S4.sup = S2.sup);

This can be modified into Todd’s division easily by adding the restriction that the parts must also belong to a common job

Steve Kass came up with a specialized version that depends on using a numeric code Assume we have a table that tells us which players are on which teams

CREATE TABLE TeamAssignments (player_id INTEGER NOT NULL REFERENCES Players(player_id)

ON DELETE CASCADE

ON UPDATE CASCADE, team_id CHAR(5) NOT NULL REFERENCES Teams(team_id)

ON DELETE CASCADE

ON UPDATE CASCADE, PRIMARY KEY (player_id, team_id));

To get pairs of players on the same team:

SELECT P1.player_id, P2.player_id FROM Players AS P1, Players AS P2 WHERE P1.player_id < P2.player_id GROUP BY P1.player_id, P2.player_id HAVING P1.player_id + P2.player_id = ALL (SELECT SUM(P3.player_id) FROM TeamAssignments AS P3 WHERE P3.player_id IN (P1.player_id, P2.player_id) GROUP BY P3.team_id);

Trang 2

19.2.5 Division with JOINs

Standard SQL has several JOIN operators that can be used to perform a relational division To find the pilots who can fly the same planes as Higgins, use this query:

SELECT SP1.Pilot

FROM (((SELECT plane FROM Hangar) AS H1

INNER JOIN

(SELECT pilot, plane FROM PilotSkills) AS SP1

ON H1.plane = SP1.plane)

INNER JOIN (SELECT *

FROM PilotSkills

WHERE pilot = 'Higgins') AS H2

ON H2.plane = H1.plane)

GROUP BY Pilot

HAVING COUNT(*) >= (SELECT COUNT(*)

FROM PilotSkills

WHERE pilot = 'Higgins');

The first JOIN finds all of the planes in the hangar for which we have

a pilot The next JOIN takes that set and finds which of those match up with (SELECT * FROM PilotSkills WHERE pilot =

'Higgins') skills The GROUP BY clause will then see that the

intersection we have formed with the joins has at least as many elements

as Higgins has planes The GROUP BY also means that the SELECT DISTINCT can be replaced with a simple SELECT If the theta operator

in the GROUP BY clause is changed from >= to =, the query finds an exact division If the theta operator in the GROUP BY clause is changed from >= to <= or <, the query finds those pilots whose skills are a superset or a strict superset of the planes that Higgins flies.

It might be a good idea to put the divisor into a VIEW for readability

in this query and as a clue to the optimizer to calculate it once Some products will execute this form of the division query faster than the nested subquery version, because they will use the PRIMARY KEY

information to precompute the joins between tables.

19.2.6 Division with Set Operators

The Standard SQL set difference operator, EXCEPT , can be used to write

a very compact version of Dr Codd’s relational division The EXCEPT

operator removes the divisor set from the dividend set If the result is

Trang 3

empty, we have a match; if there is anything left over, it has failed Using the pilots-and-hangar-tables example, we would write:

SELECT DISTINCT Pilot FROM PilotSkills AS P1 WHERE (SELECT plane FROM Hangar EXCEPT

SELECT plane FROM PilotSkills AS P2 WHERE P1.pilot = P2.pilot) IS NULL;

Again, informally, you can imagine that we got a skill list from each pilot, walked over to the hangar, and crossed off each plane he could fly

If we marked off all the planes in the hangar, we would keep this guy Another trick is that an empty subquery expression returns a NULL , which is how we can test for an empty set The WHERE clause could just

as well have used a NOT EXISTS() predicate instead of the IS NULL

predicate.

19.3 Romley’s Division

This somewhat complicated relational division is due to Richard Romley

at Salomon Smith Barney The original problem deals with two tables The first table has a list of managers and the projects they can manage The second table has a list of Personnel, their departments, and the projects to which they are assigned Each employee is assigned to one and only one department, and each employee works on one and only one project at a time But a department can have several different projects at the same time, and a single project can span several departments.

CREATE TABLE MgrProjects (mgr_name CHAR(10) NOT NULL, project_id CHAR(2) NOT NULL, PRIMARY KEY(mgr_name, project_id));

INSERT INTO Mgr_Project VALUES ('M1', 'P1'), ('M1', 'P3'), ('M2', 'P2'), ('M2', 'P3'), ('M3', 'P2'),

('M4', 'P1'), ('M4', 'P2'), ('M4', 'P3');

Trang 4

CREATE TABLE Personnel

(emp_id CHAR(10) NOT NULL,

dept CHAR(2) NOT NULL,

project_id CHAR(2) NOT NULL,

UNIQUE (emp_id, project_id),

UNIQUE (emp_id, dept),

PRIMARY KEY (emp_id, dept, project_id));

load department #1 data

INSERT INTO Personnel

VALUES ('Al', 'D1', 'P1'),

('Bob', 'D1', 'P1'),

('Carl', 'D1', 'P1'),

('Don', 'D1', 'P2'),

('Ed', 'D1', 'P2'),

('Frank', 'D1', 'P2'),

('George', 'D1', 'P2');

VALUES ('Harry', 'D2', 'P2'),

('Jack', 'D2', 'P2'),

('Larry', 'D2', 'P2'),

('Mike', 'D2', 'P2'),

('Nat', 'D2', 'P2');

VALUES ('Oscar', 'D3', 'P2'),

('Pat', 'D3', 'P2'),

('Rich', 'D3', 'P3');

The problem is to generate a report showing for each manager of each department whether is he qualified to manage none, some, or all of the projects being worked on within the department To find who can manage some, but not all, of the projects, use a version of relational division:

SELECT M1.mgr_name, P1.dept_name

FROM MgrProjects AS M1

CROSS JOIN

Trang 5

Personnel AS P1 WHERE M1.project_id = P1.project_id GROUP BY M1.mgr_name, P1.dept_name HAVING COUNT(*) <> (SELECT COUNT(emp_id) FROM Personnel AS P2 WHERE P2.dept_name = P1.dept_name);

The query is simply a relational division with <> instead of = in the

HAVING clause Richard came back with a modification of my answer that uses a characteristic function inside a single aggregate function.

SELECT DISTINCT M1.mgr_name, P1.dept_name FROM (MgrProjects AS M1

INNER JOIN Personnel AS P1

ON M1.project_id = P1.project_id) INNER JOIN

Personnel AS P2

ON P1.dept_name = P2.dept_name GROUP BY M1.mgr_name, P1.dept_name, P2.project_id HAVING MAX (CASE WHEN M1.project_id = P2.project_id THEN 1 ELSE 0 END) = 0;

This query uses a characteristic function while my original version compares a count of Personnel under each manager to a count of Personnel under each project_id The use of GROUP BY

M1.mgr_name, P1.dept_name, P2.project_id with the SELECT DISTINCT M1.mgr_name, P1.dept_name is really the tricky part in

this new query What we have is a three-dimensional space with the (x, y,

z) axis representing (mgr_name, dept_name, project_id), and then we

reduce it to two dimensions (mgr_name, dept) by seeing if Personnel on shared project_ids cover the department or not.

That observation leads to the next changes We can build a table that shows each combination of manager, department, and the level of authority they have over the projects they have in common That is the derived table T1 in the following query; authority = 1 means the manager is not on the project and authority = 2 means that he is on the project_id.

Trang 6

SELECT T1.mgr_name, T1.dept_name,

CASE SUM(T1.authority)

WHEN 1 THEN 'None'

WHEN 2 THEN 'All'

WHEN 3 THEN 'Some'

ELSE NULL END AS power

FROM (SELECT DISTINCT M1.mgr_name, P1.dept_name,

MAX (CASE WHEN M1.project_id = P1.project_id THEN 2 ELSE 1 END) AS authority

FROM MgrProjects AS M1

CROSS JOIN

Personnel AS P1

GROUP BY m.mgr_name, P1.dept_name, P1.project_id) AS T1 GROUP BY T1.mgr_name, T1.dept_name;

Another version, using the airplane hangar example:

SELECT PS1.pilot,

CASE WHEN COUNT(PS1.plane) >

(SELECT COUNT(plane) FROM Hanger)

AND COUNT(H1.plane) =

(SELECT COUNT(plane)FROM Hanger)

THEN 'more than all'

WHEN COUNT(PS1.plane) =

AND COUNT(H1.plane) =

THEN 'exactly all '

WHEN MIN(H1.plane) IS NULL

THEN 'none '

ELSE 'some ' END AS skill_level

FROM PilotSkills AS PS1

LEFT OUTER JOIN

Hanger AS H1

ON PS1.plane = H1.plane

GROUP BY PS1.pilot;

We can now sum the authority numbers for all the projects within a department to determine the power this manager has over the

department as a whole If he had a total of one, he has no authority over Personnel on any project in the department If he had a total of two, he

Trang 7

has power over all Personnel on all projects in the department If he had

a total of three, he has both a one and a two authority total on some projects within the department Here is the final answer.

Results mgr_name dept power M1 D1 Some M1 D2 None M1 D3 Some M2 D1 Some M2 D2 All M2 D3 All M3 D1 Some M3 D2 All M3 D3 Some M4 D1 All M4 D2 All M4 D3 All

19.4 Boolean Expressions in an RDBMS

Given the usual “hangar and pilots” schema, we want to create and store queries that involve Boolean expressions such as “Find the pilots who can fly a Piper Cub and also an F-14 or F-17 Fighter.” The trick is to put the expression into the disjunctive canonical form In English that means a bunch of AND ed predicates that are then OR ed together Any Boolean function can be expressed this way This form is canonical when each Boolean variable appears exactly once in each term When all variables are not required to appear in every term, the form is called a disjunctive normal form The algorithm to convert any Boolean expression into disjunctive canonical form is a bit complicated, but can

be found in a good book on circuit design Our simple example would convert to this predicate.

('Piper Cub' AND 'F-14 Fighter') OR ('Piper Cub' AND 'F-17 Fighter')

We then load the predicate into this table:

CREATE TABLE BooleanExpressions (and_grp INTEGER NOT NULL,

Trang 8

skill CHAR(10) NOT NULL,

PRIMARY KEY (and_grp, skill));

INSERT INTO BooleanExpressions VALUES (1, 'Piper Cub');

INSERT INTO BooleanExpressions VALUES (1, 'F-14 Fighter'); INSERT INTO BooleanExpressions VALUES (2, 'Piper Cub');

INSERT INTO BooleanExpressions VALUES (2, 'F-17 Fighter');

Assume we have a table of job candidates:

CREATE TABLE Candidates

(candidate_name CHAR(15) NOT NULL,

skill CHAR(10) NOT NULL,

PRIMARY KEY (candidate_name, skill));

INSERT INTO Candidates VALUES ('John', 'Piper Cub'); winner

INSERT INTO Candidates VALUES ('John', 'B-52 Bomber');

INSERT INTO Candidates VALUES ('Mary', 'Piper Cub'); winner

INSERT INTO Candidates VALUES ('Mary', 'F-17 Fighter');

INSERT INTO Candidates VALUES ('Larry', 'F-14 Fighter'); winner INSERT INTO Candidates VALUES ('Larry', 'F-17 Fighter');

INSERT INTO Candidates VALUES ('Moe', 'F-14 Fighter'); winner

INSERT INTO Candidates VALUES ('Moe', 'F-17 Fighter');

INSERT INTO Candidates VALUES ('Moe', 'Piper Cub');

INSERT INTO Candidates VALUES ('Celko', 'Piper Cub'); loser

INSERT INTO Candidates VALUES ('Celko', 'Blimp');

INSERT INTO Candidates VALUES ('Smith', 'Kite'); loser

INSERT INTO Candidates VALUES ('Smith', 'Blimp');

The query is simple now:

SELECT DISTINCT C1.candidate_name

FROM Candidates AS C1, BooleanExpressions AS Q1

WHERE C1.skill = Q1.skill

GROUP BY Q1.and_grp, C1.candidate_name

HAVING COUNT(C1.skill)

= (SELECT COUNT(*)

FROM BooleanExpressions AS Q2

WHERE Q1.and_grp = Q2.and_grp);

Trang 9

You can retain the COUNT() information to rank candidates For example, Moe meets both qualifications, while other candidates meet only one of the two

19.5 FIFO and LIFO Subsets

This will be easier to explain with an example for readers who have not worked with an Inventory system before Imagine that we have a warehouse of one product to which we add stock once a day.

CREATE TABLE InventoryReceipts (receipt_nbr INTEGER PRIMARY KEY, purchase_date DATETIME NOT NULL, qty_on_hand INTEGER NOT NULL CHECK (qty_on_hand >= 0), unit_price DECIMAL (12,4) NOT NULL);

Let’s use this sample data for discussion.

InventoryReceipts receipt_nbr purchase_date qty_on_hand unit_price ========================================

1 '2006-01-01' 15 10.00

2 '2006-01-02' 25 12.00

3 '2006-01-03' 40 13.00

4 '2006-01-04' 35 12.00

5 '2006-01-05' 45 10.00

The business now sells 100 units on 2006-01-05 How do you calculate the value of the stock sold? There is not one right answer, but here are some options:

1 Use the current replacement cost, which is $10.00 per unit as

of January 5, 2006 That would mean the sale cost us

$1,000.00 because of a recent price break.

2 Use the current average price per unit We have a total of 160 units, for which we paid a total of $1,840.00, and that gives us

an average cost of $11.50 per unit, or $1,150.00 in total inventory costs.

Trang 10

3 LIFO , which stands for “Last In, First Out.” We start by looking

at the most recent purchases and work backwards through time.

2006-01-05: 45 * $10.00 = $450.00 and 45 units

2006-01-04: 35 * $12.00 = $420.00 and 80 units

2006-01-03: 20 * $13.00 = $260.00 and 100 with 20 units left over

for a total of $1,130.00 in inventory costs.

4 FIFO , which stands for “First In, First Out.” We start by

looking at the earliest purchases and work forward through time.

2006-01-01: 15 * $10.00 = $150.00 and 15 units

2006-01-02: 25 * $12.00 = $300.00 and 40 units

2006-01-03: 40 * $13.00 = $520.00 and 80 units

2006-01-04: 20 * $12.00 = $240.00 with 15 units left over

for a total of $1,210.00 in inventory costs.

The first two scenarios are trivial to program The LIFO and FIFO are more interesting because they involve matching the order against blocks

of inventory in a particular order Consider this view:

CREATE VIEW LIFO (stock_date, unit_price, tot_qty_on_hand, tot_cost)

AS

SELECT R1.purchase_date, R1.unit_price, SUM(R2.qty_on_hand), SUM(R2.qty_on_hand *

R2.unit_price)

FROM InventoryReceipts AS R1,

InventoryReceipts AS R2

WHERE R2.purchase_date >= R1.purchase_date

GROUP BY R1.purchase_date, R1.unit_price;

A row in this view tells us the total quantity on hand, the total cost of the goods in inventory, and what we were paying for items on each date The quantity on hand is a running total We can get the LIFO cost with this query:

Định dạng
Số trang	10
Dung lượng	124,77 KB