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DBDI/ Lecture 9 Relational DB Languages Advanced SQL
Dr. Ala Al-Zobaidie The slides are based on the textbook Database Systems by Connolly & Begg
30/05/2007 DBDI / AdvSQL 2
Lecture’s Objectives /1 - SQL
- Use aggregate functions.
- Group data using GROUP BY and HAVING.
- Use subqueries.
- ANY and ALL
- Joining tables
- Computing joins
- Outer Joins
- Exist and Not Exist
- Set operations (Union, Intersect and Except)
- How to update database using INSERT, UPDATE, and DELETE.
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SELECT Statement - Aggregates
- ISO standard defines five aggregate functions:
COUNT returns number of values in specified column.
SUM returns sum of values in specified column.
AVG returns average of values in specified column.
MIN returns smallest value in specified column.
MAX returns largest value in specified column.
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SELECT Statement - Aggregates
- Each operates on a single column of a table and return single value.
- COUNT, MIN, and MAX apply to numeric and non-numeric fields, but SUM and AVG may be used on numeric fields only.
- Apart from COUNT(*), each function eliminates nulls first and operates only on remaining non-null values.
SELECT Statement - Aggregates
- COUNT(*) counts all rows of a table, regardless of whether nulls or duplicate values occur.
- Can use DISTINCT before column name to eliminate duplicates.
- DISTINCT has no effect with MIN/MAX (it eliminates duplicates itself),
- but may have with SUM/AVG, e.g.
- select avg(sal) from emp ≠ select avg(distinct sal) from emp;
SELECT Statement - Aggregates
- Aggregate functions can be used only in SELECT list and in HAVING clause.
- If SELECT list includes an aggregate function and there is no GROUP BY clause, SELECT list cannot reference a column outwith an aggregate function. For example, following is illegal:
SELECT staffNo, COUNT(salary) FROM Staff;
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Example 5.13 Use of COUNT()*
How many properties cost more than 350 per month to rent?
SELECT COUNT(*) AS count FROM PropertyForRent WHERE rent > 350;
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Example 5.14 Use of COUNT(DISTINCT)
How many different properties viewed in May ‘01?
SELECT COUNT(DISTINCT propertyNo) AS count FROM Viewing WHERE date BETWEEN ‘1-May-01’ AND '31-May-01';
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Example 5.15 Use of COUNT and SUM
Find number of Managers and sum of their salaries. SELECT COUNT(staffNo) AS count, SUM(salary) AS sum FROM Staff WHERE position = 'Manager';
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Example 5.16 Use of MIN, MAX, AVG
Find minimum, maximum, and average staff salary.
SELECT MIN(salary) AS min, MAX(salary) AS max, AVG(salary) AS avg FROM Staff;
SELECT Statement - Grouping
- Use GROUP BY clause to get sub-totals.
- SELECT and GROUP BY closely integrated: each item in SELECT list must be single-valued per group , and SELECT clause may only contain: - Column names - Aggregate functions - Constants - Expression involving combinations of the above.
SELECT Statement - Grouping
- All column names in SELECT list must appear in GROUP BY clause unless name is used only in an aggregate function.
- If WHERE is used with GROUP BY, WHERE is applied first, then groups are formed from remaining rows satisfying predicate.
- ISO considers two nulls to be equal for purposes of GROUP BY.
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Example 5.19 Subquery with Equality
List staff who work in branch at '163 Main St'.
SELECT staffNo, fName, lName, position FROM Staff WHERE branchNo = (SELECT branchNo FROM Branch WHERE street = '163 Main St');
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Example 5.19 Subquery with Equality
- Inner SELECT finds branch number for branch at '163 Main St' ('B003').
- Outer SELECT then retrieves details of all staff who work at this branch.
- Outer SELECT then becomes:
SELECT staffNo, fName, lName, position FROM Staff WHERE branchNo = 'B003';
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Example 5.19 Subquery with Equality
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Example 5.20 Subquery with Aggregate
List all staff whose salary is greater than the average salary, and show by how much.
SELECT staffNo, fName, lName, position, salary – (SELECT AVG(salary) FROM Staff) As SalDiff FROM Staff WHERE salary > (SELECT AVG(salary) FROM Staff);
Example 5.20 Subquery with Aggregate
- Cannot write 'WHERE salary > AVG(salary)'
- Instead, use subquery to find average salary (17000), and then use outer SELECT to find those staff with salary greater than this:
SELECT staffNo, fName, lName, position, salary – 17000 As salDiff FROM Staff WHERE salary > 17000;
Example 5.20 Subquery with Aggregate
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Subquery Rules
- ORDER BY clause may not be used in a subquery (although it may be used in outermost SELECT).
- Subquery SELECT list must consist of a single column name or expression, except for subqueries that use EXISTS.
- By default, column names refer to table name in FROM clause of subquery. Can refer to a table in FROM using an alias. 30/05/2007 DBDI / AdvSQL 26
Subquery Rules
- When subquery is an operand in a comparison, subquery must appear on right-hand side.
- A subquery may not be used as an operand in an expression.
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Example 5.21 Nested subquery: use of IN
List properties handled by staff at '163 Main St'.
SELECT propertyNo, street, city, postcode, type, rooms, rent FROM PropertyForRent WHERE staffNo IN (SELECT staffNo FROM Staff WHERE branchNo = (SELECT branchNo FROM Branch WHERE street = '163 Main St')); 30/05/2007 DBDI / AdvSQL 28
Example 5.21 Nested subquery: use of IN
ANY and ALL
- ANY and ALL may be used with subqueries that produce a single column of numbers.
- With ALL, condition will only be true if it is satisfied by all values produced by subquery.
- With ANY, condition will be true if it is satisfied by any values produced by subquery.
- If subquery is empty, ALL returns true, ANY returns false.
- SOME may be used in place of ANY.
Example 5.22 Use of ANY/SOME
Find staff whose salary is larger than salary of at least one member of staff at branch B003.
SELECT staffNo, fName, lName, position, salary FROM Staff WHERE salary > SOME (SELECT salary FROM Staff WHERE branchNo = 'B003');
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Example 5.24 Simple Join
- Only those rows from both tables that have identical values in the clientNo columns (c.clientNo = v.clientNo) are included in result.
- Equivalent to equi-join in relational algebra.
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Alternative JOIN Constructs
- SQL provides alternative ways to specify joins:
FROM Client c JOIN Viewing v ON c.clientNo = v.clientNo FROM Client JOIN Viewing USING clientNo FROM Client NATURAL JOIN Viewing
- In each case, FROM replaces original FROM and WHERE. However, first produces table with two identical clientNo columns.
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Example 5.25 Sorting a join
For each branch, list numbers and names of staff who manage properties, and properties they manage.
SELECT s.branchNo, s.staffNo, fName, lName, propertyNo FROM Staff s, PropertyForRent p WHERE s.staffNo = p.staffNo ORDER BY s.branchNo, s.staffNo, propertyNo; 30/05/2007 DBDI / AdvSQL 40
Example 5.25 Sorting a join
Example 5.26 Three Table Join
For each branch, list staff who manage properties, including city in which branch is located and properties they manage.
SELECT b.branchNo, b.city, s.staffNo, fName, lName, propertyNo FROM branch b, staff s, property_for_rent p WHERE b.branchNo = s.branchNo AND s.staffNo = p.staffNo ORDER BY b.branchNo, s.staffNo, propertyNo;
Example 5.26 Three Table Join
- Alternative formulation for FROM and WHERE: FROM (branch b JOIN Staff s USING branchNo) AS bs JOIN PropertyForRent p USING staffNo
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Example 5.27 Multiple Grouping Columns Find number of properties handled by each staff member.
SELECT s.branchNo, s.staffNo, COUNT(*) AS count FROM Staff s, PropertyForRent p WHERE s.staffNo = p.staffNo GROUP BY s.branchNo, s.staffNo ORDER BY s.branchNo, s.staffNo;
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Example 5.27 Multiple Grouping Columns
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Computing a Join
Procedure for generating results of a join are:
- Form Cartesian product of the tables named in FROM clause.
- If there is a WHERE clause, apply the search condition to each row of the product table, retaining those rows that satisfy the condition.
- For each remaining row, determine value of each item in SELECT list to produce a single row in result table.
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Computing a Join
- If DISTINCT has been specified, eliminate any duplicate rows from the result table.
- If there is an ORDER BY clause, sort result table as required.
- SQL provides special format of SELECT for Cartesian product: SELECT [DISTINCT | ALL] {* | columnList} FROM Table1 CROSS JOIN Table
Outer Joins
- If one row of a joined table is unmatched, row is omitted from result table.
- Outer join operations retain rows that do not satisfy the join condition.
- Consider following tables:
Outer Joins
- The (inner) join of these two tables: SELECT b., p. FROM Branch1 b, PropertyForRent1 p WHERE b.bCity = p.pCity;
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Example 5.30 Full Outer Join
- Includes rows that are unmatched in both tables.
- Unmatched columns are filled with NULLs.
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EXISTS and NOT EXISTS
- EXISTS and NOT EXISTS are for use only with subqueries.
- Produce a simple true/false result.
- True if and only if there exists at least one row in result table returned by subquery.
- False if subquery returns an empty result table.
- NOT EXISTS is the opposite of EXISTS.
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EXISTS and NOT EXISTS
- As (NOT) EXISTS check only for existence or non-existence of rows in subquery result table, subquery can contain any number of columns.
- Common for subqueries following (NOT) EXISTS to be of form: (SELECT * ...)
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Example 5.31 Query using EXISTS
Find all staff who work in a London branch.
SELECT staffNo, fName, lName, position FROM Staff s WHERE EXISTS (SELECT * FROM Branch b WHERE s.branchNo = b.branchNo AND city = 'London');
Example 5.31 Query using EXISTS Example 5.31 Query using EXISTS
- Note, search condition s.branchNo = b.branchNo is necessary to consider correct branch record for each member of staff.
- If omitted, would get all staff records listed out because subquery: SELECT * FROM Branch WHERE city='London'
- would be always be true and query would be: SELECT staffNo, fName, lName, position FROM Staff WHERE true;
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Example 5.31 Query using EXISTS
- Could also write this query using join construct: SELECT staffNo, fName, lName, position FROM Staff s, Branch b WHERE s.branchNo = b.branchNo AND city = 'London';
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Union, Intersect, and Difference (Except)
- Can use normal set operations of Union, Intersection, and Difference to combine results of two or more queries into a single result table.
- Union of two tables, A and B, is table containing all rows in either A or B or both.
- Intersection is table containing all rows common to both A and B.
- Difference is table containing all rows in A but not in B.
- Two tables must be union compatible.
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Union, Intersect, and Difference (Except)
- Format of set operator clause in each case is:
op [ALL] [CORRESPONDING [BY {column1 [, ...]}]]
- If CORRESPONDING BY specified, set operation performed on the named column(s).
- If CORRESPONDING specified but not BY clause, operation performed on common columns.
- If ALL specified, result can include duplicate rows. 30/05/2007 DBDI / AdvSQL 64
Union, Intersect, and Difference (Except)
Example 5.32 Use of UNION
List all cities where there is either a branch or property.
(SELECT city FROM Branch WHERE city IS NOT NULL) UNION (SELECT city FROM PropertyForRent WHERE city IS NOT NULL);
Example 5.32 Use of UNION
(SELECT * FROM Branch WHERE city IS NOT NULL) UNION CORRESPONDING BY city (SELECT * FROM PropertyForRent WHERE city IS NOT NULL);
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INSERT
INSERT INTO TableName [ (columnList) ] VALUES (dataValueList)
- columnList is optional; if omitted, SQL assumes a list of all columns in their original CREATE TABLE order.
- Any columns omitted must have been declared as NULL when table was created, unless DEFAULT was specified when creating column. 30/05/2007 DBDI / AdvSQL 74
INSERT
- dataValueList must match columnList as follows: - number of items in each list must be same; - must be direct correspondence in position of items in two lists; - data type of each item in dataValueList must be compatible with data type of corresponding column.
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Example 5.35 INSERT … VALUES
Insert a new row into Staff table supplying data for all columns.
INSERT INTO Staff VALUES ('SG16', 'Alan', 'Brown', 'Assistant', 'M', Date‘1957-05-25', 8300, 'B003');
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Example 5.36 INSERT using Defaults
Insert a new row into Staff table supplying data for all mandatory columns. INSERT INTO Staff (staffNo, fName, lName, position, salary, branchNo) VALUES ('SG44', 'Anne', 'Jones', 'Assistant', 8100, 'B003');
- Or INSERT INTO Staff VALUES ('SG44', 'Anne', 'Jones', 'Assistant', NULL, NULL, 8100, 'B003');
INSERT … SELECT
- Second form of INSERT allows multiple rows to be copied from one or more tables to another:
INSERT INTO TableName [ (columnList) ] SELECT ...
Example 5.37 INSERT … SELECT
Assume there is a table StaffPropCount that contains names of staff and number of properties they manage:
StaffPropCount(staffNo, fName, lName, propCnt)
Populate StaffPropCount using Staff and PropertyForRent tables.
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Example 5.37 INSERT … SELECT INSERT INTO StaffPropCount (SELECT s.staffNo, fName, lName, COUNT(*) FROM Staff s, PropertyForRent p WHERE s.staffNo = p.staffNo GROUP BY s.staffNo, fName, lName) UNION (SELECT staffNo, fName, lName, 0 FROM Staff WHERE staffNo NOT IN (SELECT DISTINCT staffNo FROM PropertyForRent)); 30/05/2007 DBDI / AdvSQL 80
Example 5.37 INSERT … SELECT
- If second part of UNION is omitted, excludes those staff who currently do not manage any properties.
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UPDATE
UPDATE TableName SET columnName1 = dataValue [, columnName2 = dataValue2...] [WHERE searchCondition]
- TableName can be name of a base table or an updatable view.
- SET clause specifies names of one or more columns that are to be updated.
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UPDATE
- WHERE clause is optional:
- if omitted, named columns are updated for all rows in table.;
- if specified, only those rows that satisfy searchCondition are updated.
- New dataValue(s) must be compatible with data type for corresponding column.
Example 5.38/39 UPDATE All Rows
Give all staff a 3% pay increase. UPDATE Staff SET salary = salary*1.03;
Give all Managers a 5% pay increase.
UPDATE Staff SET salary = salary*1. WHERE position = 'Manager';
Example 5.40 UPDATE Multiple Columns Promote David Ford (staffNo = 'SG14') to Manager and change his salary to 18,000.
UPDATE Staff SET position = 'Manager', salary = 18000 WHERE staffNo = 'SG14';
