8/13/2019 Writing Triggers
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Writing Triggers
1. Scenario
Typical component of any sales system used in supermarket chains
is a database schema for registering customer
sales. Suppose the following two relations are used to represent
the sales operations and the items sold in each of
them:
sales(invNo:INTEGER, customer:STRING, items:INTEGER,
total:MONEY, sale_date:DATEitems(!osNo:INTEGER, invno:INTEGER,
name:STRING, !rice:MONEY, "t#:INTEGER, sum:MONEY
Assuming most of the calculations done during customer sales are
implemented using database triggers, in this
seminar you should write and experiment with various triggers
for automation of certain typical operations for sales.
2. Aims of this seminar
To practice writing of row and statement triggers against
database tables using standard SQ and to explore the
effect of their execution using interactive database
manipulation
To analy!e the reasons for table mutation when using database
triggers and the restrictions for avoiding them
3. Tasks
a" #rite SQ $$ statements for creating database tables which
implement the above database schema and
execute them using SQ%&lus under your database account
'Solution:given in salesdata$ase%s"lfile"
b" #rite row trigger against ITEMStable, which calculates
automatically the sum of all bought items using theunit price and
the (uantity for each of them, and another statement trigger
against the above tables which
automatically updates the total sum and the number of different
positions bought during the current saleoperation) execute them
against the same account 'Solution:given in
salesro&tri''ers%s"lfile"
c" &repare and load some test data for two different sales
operations with several item positions and explore theresult of
automatic trigger execution after each of them 'Solution:given in
saleso!erations%s"lfile"
d" *reate an audit table HISTfor registering different possible
sales operations performed typically by the
cashiers at the till according to the following
specification:
ist(no:INTEGER,ta$_name:STRING,s"l_o!:)*AR,dat:DATE,col_name:STRING,count:INTEGER
where s"l_o!can have one of the two possible values + ++ for
updating of prices and (uantities and +D+ -
for deleting of some of the positions in the current sales
operation or canceling the entire operation 'Solution:
given in salesaudit%s"lfile"
e" #rite some statement triggers against the two tables SALESand
ITEMSwhich register in HISTtable the
corresponding operations performed at the till and execute them
under the same account 'Solution:given in
salesstatementtri''ers%s"lfile"
f" *hange item prices, (uantities, positions and sales summaries
in the existing sales operations 'Solution:given in
salescan'es%s"l file". Analy!e and explain the generated
messages.
g" xecute the file salesmutatin'ta$les%s"lunder the same account
and explore its content 'Hint: it contains
row triggers which could cause table mutation when executed"
