SQL: Interactive Queries (2) Prof. Weining Zhang Cs.utsa.edu
Aggregate Functions Functions that take a set of tuples and compute an aggregated value. Five standard functions: count, min, max, avg, sum They ignore null values. Find the total number, the average, minimum, and maximum GPA of students whose age is 17. select count(*), avg(gpa), min(gpa), max(gpa) from Students where Age = 17 Lecture 12 SQL: Interactive Queries (2) 2
Aggregate Functions (cont.) Find id and name of students who take 5 or more courses. select SID, Name from Students s where 5 <= (select count(distinct Cno) from Enrollment where SID = s.sid) Count(distinct Cno) distinct count(cno). Why? Must make sure the subquery generates a value comparable in the predicate. Lecture 12 SQL: Interactive Queries (2) 3
Group By Clause List id and name of students together with the number of hours still needed to graduate, assuming 120 hours are required. select s.sid, Name, 120 - sum(hours) Hours-Needed from Students s, Enrollment e, Courses c where s.sid = e.sid and e.cno = c.cno and Grade <= C group by s.sid, Name Enrolled courses are grouped by students. Lecture 12 SQL: Interactive Queries (2) 4
Group By Clause (cont.) Aggregate functions often applied to groups. One tuple is generated per group When using group by, select clause can contain only grouping attributes and aggregate func. Every grouping attribute must be in the select clause.the following is an illegal query (why?): select Age, SID, avg(gpa) from Students group by Age Lecture 12 SQL: Interactive Queries (2) 5
Having Clause For each student age group with more than 50 members, list the age and the number of students with that age. select Age, count(*) from Students group by Age having count(*) > 50 Conditions on aggregate functions are specified in the having clause. Select & Having may have different functions. Lecture 12 SQL: Interactive Queries (2) 6
Order By Clause List student names in ascending order. select Name from Students order by Name asc The default is ascending order. List students with GPA higher than 3.5, first in descending order of GPA, and then in ascending order of name. select * from Students where GPA > 3.5 order by GPA desc, Name asc Lecture 12 SQL: Interactive Queries (2) 7
Some Complex Queries Find the average number of CS courses a student takes. For non-cs major students who take more CS courses than he does with his major courses, and have taken at lease 2 CS courses, list their id, name, number of CS courses, number of major courses, sorted first in descending order of number of CS courses, then in ascending order of name. Lecture 12 SQL: Interactive Queries (2) 8
Interactive SQL Summary A query may have six clauses: select, from, where, group by, having, order by. Conceptual evaluation of the query: 1. Evaluate From (cross product) 2. Evaluate Where (selection) 3. Evaluate Group By (form groups) 4. Evaluate Aggregate functions on groups 5. Evaluate Having (choose groups to output) 6. Evaluate Order By (sorting) 7. Evaluate remaining Select (projection) Lecture 12 SQL: Interactive Queries (2) 9
Interactive SQL Summary (count.) Many ways to express a query. Flat queries may be more efficient. Nested queries may be easier to understand. Duplicate elimination may be costly. <> (not equal) at predicate level often gives a wrong answer. Use set difference, not in, not exists, etc. instead. Need to handle null values explicitly. DBMSs often provide many convenient functions. But need to check the compatibility. Lecture 12 SQL: Interactive Queries (2) 10
Expressive Power of SQL SQL is relational complete. Can express any relational algebraic query. SQL is more powerful then relational algebra. Can express aggregation, ordering, recursion, etc. SQL is not computational complete. Can not do everything a general programming language can do. Lecture 12 SQL: Interactive Queries (2) 11
Create Table Re-visited Can combine table creation with insertion of tuples using a query. create table Full-Professors as select FID, Name, Office from Faculty where Rank = Full Professor Lecture 12 SQL: Interactive Queries (2) 12
Update By Queries Relation: Top_Students (SID, Name, GPA) Insert students with a GPA 3.8 or higher into the Top_Students table. insert into Top_Students select SSN, Name, GPA from Students where GPA >= 3.8 Delete all students who take no courses. delete from Students where SID not in (select SID from Enrollment) Lecture 12 SQL: Interactive Queries (2) 13
Update Statement For every student who takes Database I, set the Grade to A. update Enrollment set Grade = 'A' where Cno in (select Cno from Courses where Title = Database I') Lecture 12 SQL: Interactive Queries (2) 14
Truncate vs Delete * Use delete to remove data and keep the table storage space. delete from Departments; Use truncate to remove data and release table storage space. truncate table Departments; Lecture 12 SQL: Interactive Queries (2) 15
Views A view is a virtual table (as opposed to stored base table) defined by a query, directly or indirectly, on base tables. create view Top_Students as select SSN, Name, GPA from Students where GPA >= 3.8 A view may be defined in terms of other views. Lecture 12 SQL: Interactive Queries (2) 16
Views (cont.) The query in view definition is usually not executed until the view is queried. Typically, no data is stored for a view. A view is queried as if it is a base table. Find name and GPA of top students whose name starts with a `K'. select Name, GPA from Top_Students where Name like 'K%' Lecture 12 SQL: Interactive Queries (2) 17
Query Modification Queries on a view are translated into queries on base tables by folding the view. Previous query is translated first into: select Name, GPA from (select SSN, Name, GPA from Students where GPA >= 3.8) where Name like 'K% Then into select Name, GPA from Students where GPA >= 3.8 and Name like 'K%' Lecture 12 SQL: Interactive Queries (2) 18
Why Use Views? Data independence: keep existing application programs from changes of base table schemas. Access control: provide a mechanism for hiding sensitive data from certain users. Productivity improvement: make user queries easier to express. Lecture 12 SQL: Interactive Queries (2) 19
Example of Using Views Consider following base tables and a view: Students (SID, Name, Birthday, GPA, Phone) Emrollment(SID, Cno, Grade) Courses(Cno, Title, Hours, Dept) create view Student-Course as select SID, Name, Age(Birthday) Age, GPA, c.cno, Title from Students s, Enrollment e, Courses c where s.sid=e.sid and e.cno = c.cno Lecture 12 SQL: Interactive Queries (2) 20
Example of Using Views (cont.) Data independence: Applications using the view are not affected if Age is stored or derived. Access control: Phone and Birthday of students are hidden from users. Productivity improvement: Find all courses taken by a given student is much simpler: select Cno, Title from Student_Course where SID = X Lecture 12 SQL: Interactive Queries (2) 21
Views and Updates What should happen if a user changes the data in the Student-Course view? insert into Student-Course values (1234, Dave Hall, 32, 3.15, CS334, B ) A view can not be updated if Contains group by and aggregate functions Involves multiple tables A single-table view can be updated if it contains a key of the table Lecture 12 SQL: Interactive Queries (2) 22
View Update Example * Which student should be deleted? create view Age_distribution as select Age, count(*) TotalNo from Students group by Age update Age_distribution set TotalNo = TotalNo 1 where Age = 20 Which base relation should be changed? delete from Student_Course where SID = '1234' Lecture 12 SQL: Interactive Queries (2) 23
Maintaining Materialized Views One may want to materialize a view (i.e., run its definition query and store the result) as is commonly done in industry (data warehouse). (Why?) View Maintenance: How to maintain the consistency between a view and its base tables, when base tables are updated? Incremental View Maintenance: How to maintain a view without re-computing the entire view? Lecture 12 SQL: Interactive Queries (2) 24