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SQL constrains and keys

SQL constrains and keys. SORTED RESULTS. Sort the results by a specified criterion SELECT columns FROM tables WHERE predicates ORDER BY column ASC/DESC; select fname, sid from students order by fname ASC;. Constraints. not null validate conditions for fields Unique

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SQL constrains and keys

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  1. SQL constrains and keys

  2. SORTED RESULTS • Sort the results by a specified criterion • SELECT columnsFROM tablesWHERE predicatesORDER BY column ASC/DESC; • select fname, sid from students order by fname ASC;

  3. Constraints • not null • validate conditions for fields • Unique create table students ( sid char(5) not null unique, fname varchar(20), lname varchar(20) not null, minit char, age int(3) check (age>0), fatherage int(3) check (fatherage>0), check (fatherage>age)) ;

  4. Exercise • Try to insert at least two different records that would violate at least one constraint.

  5. Primary keys • a primary key is a value that can be used to identify a particular row in a table. • A super key for a relation scheme R is any subset k of R that satisfies the property that in any valid relation under the scheme R it is not possible to have two different tuples with the same values under k. • A candidate key for R is any super key for R such as none of its proper subsets is also a super key • A primary key is one of the candidate keys chosen by the designer of the database

  6. Primary key examples • What would happen if more than one bank account would have the same number ? • Solution : unique ids ? • How to enforce: primary keys • Why not only “unique” : • primary keys are automatically indexed by the dbms • primary keys are not null • if a column is unique than more than one null row with data can exist create table students ( sid varchar(5), fname varchar(20), lname varchar(20) not null, minit char, primary key (sid));

  7. The primary key may be a tuple create table enrolls ( sid varchar(5), term varchar(10), lineno int(4), primary key (sid,term,lineno));

  8. Foreign keys • A foreign key is a field or group of fields in a database record that point to a key field or group of fields forming a key of another database record in some (usually different) table. Usually a foreign key in one table refers to the primary key of another table create table enrolls ( sid varchar(5) not null, term varchar(10), lineno int(4), primary key (sid,term,lineno), foreign key (sid) references students);

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