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Page 48 of 50
Difference Between Trigger and Procedure
In this post, we will understand the difference between trigger and a procedure.TriggersIt is implicitly invoked when an event such as INSERT, DELETE, and UPDATE occurs in a table of a database.Nesting of triggers can be achieved using a table.A trigger can’t be called or defined inside another trigger.Transactional statements such as ‘COMMIT’, ‘ROLLBACK’, ‘SAVEPOINT’ can’t be used in triggers.They are used to maintain referential integrity.This is done by keeping a record of the activities performed on a table.No values are returned in a trigger.No value can be passed as a parameter to a trigger.Syntax to define a trigger:CREATE TRIGGER ...
Read MoreDifference Between ROLAP and MOLAP
In this post, we will understand the difference between ROLAP and MOLAP.ROLAPIt stands for Relational Online Analytical Processing.It is used for large volumes of data.The access time in ROLAP is slow.It stores data in the form of relation tables.The data in ROLAP is fetched from a data warehouse.It uses complex SQL queries.A static multidimensional view of the data is created in ROLAP.MOLAPIt stands for Multidimensional Online Analytical Processing.It is used for less/limited volumes of data.The access time is quick in MOLAP.Data is stored in a multidimensional array.Data is fetched from the MDDBs database.A sparse matrix is used in MOLAP.Dynamic multidimensional ...
Read MoreDifference Between T-SQL and PL-SQL
In this post, we will understand the difference between T-SQL and PL-SQL.T-SQLIt is a Microsoft product.It is known as Transact Structure Query language.It gives a high degree of control to the developers/programmers.It works its best, and provides good performance with Microsoft SQL server.It is easy.It is simple to understand.It allows the insertion of multiple rows into a table.This is done with the help of the ‘BULK INSERT’ statement.The ‘SELECT INTO’ statement is used in T-SQLIn this, the ‘NOT EXISTS’ clause can be used with the ‘SELECT’ statements.PL-SQLIt is an Oracle product.It is known as Procedural Language Structural Query Language.It is ...
Read MoreDifference Between Grant and Revoke
In this post, we will understand the difference between grant and revoke.GrantIt is a DCL command.It grants permissions to users on database objects.It can also be used to assign access rights to users.For every user, the permissions need to be specified.When the access is decentralized, permission granting is easier.Syntax:grant privilege_name on object_name to {user_name | public | role_name}RevokeIt is a DCL command.It removes permissions if they are granted to users on database objects.It takes away/revokes the rights of the users.If access for a user is removed, all specific permissions provided by that user to others will be removed.If decentralized access ...
Read MoreWhat all constraints can be added to a PostgreSQL table?
There are 6 types of constraints that can be generally used with a PostgreSQL table. They are listed and explained below −NOT NULL CONSTRAINTThis is a very common constraint. If there is a particular column that cannot have null values, you add this constraint at the time of table creation. For example, if we create a marks table, which can’t have NULL values for the name, then the table creation command will look like −CREATE TABLE marks( name VARCHAR NOT NULL, roll_no INTEGER, marks_obtained INTEGER );Now, if we try to insert a row into this table, without ...
Read MoreCreate Primary Key on an existing table in PostgreSQL?
Although quite infrequent, you may come across situations wherein you need to define the primary key on an existing table. This can be achieved using the ALTER TABLE statement.The syntax is −ALTER TABLE table_name ADD PRIMARY KEY (column_name1, column_name2, …., columns_nameN)As can be seen from the above syntax, you can define PRIMARY KEY on multiple columns. When you have defined the PRIMARY KEY on multiple columns, the condition is that the column pairs should have unique and non-null values. Thus, if the PRIMARY KEY is defined on (column1, column2), the values (value1, value2), (value3, value2), and (value1, value4) are allowed. ...
Read MoreExtract day, hour, minute, etc. from a datetime column in PostgreSQL?
Let us create a new table containing a single timestamp column −CREATE TABLE timestamp_test( ts timestamp );Now let us populate it with some data −INSERT INTO timestamp_test(ts) VALUES(current_timestamp), (current_timestamp+interval '5 days'), (current_timestamp-interval '18 hours'), (current_timestamp+interval '1 year'), (current_timestamp+interval '3 minutes'), (current_timestamp-interval '6 years');If you query the table (SELECT * from timestamp_test), you will see the following output −ts2021-01-30 19:23:24.0080872021-02-04 19:23:24.0080872021-01-30 01:23:24.0080872022-01-30 19:23:24.0080872021-01-30 19:26:24.0080872015-01-30 19:23:24.008087Now, in order to extract hour, minute, etc. from the timestamp column, we use the EXTRACT function. Some examples are shown below −SELECT EXTRACT(HOUR from ts) as hour from timestamp_testOutput −hour19191191919Similarly −SELECT EXTRACT(MONTH from ts) as ...
Read MoreHow to combine different columns of a table to yield a single column in query output in PostgreSQL?
Suppose you have a table user_info that contains the state and district of different users. An example is given below −namedistrictstateAnilMumbaiMaharashtraJoyJhalawarRajasthanRonPuneMaharashtraReenaMeerutUttar PradeshNow, if you want to combine the state and district in a single field called location, this is how you should be able to do it −SELECT name, district || ', ' || state as location from user_infoThe || operator is the string concatenation operator. The output will be −namelocationAnilMumbai, MaharashtraJoyJhalawar, RajasthanRonPune, MaharashtraReenaMeerut, Uttar PradeshSimilar operations can also be performed on numerical values. Suppose you have a table marks containing the total marks scored by students and the maximum ...
Read MoreHow to look for partial string matches in queries in PostgreSQL?
Suppose you have a table user_info containing the names of users and their addresses. An example is given below −nameaddressAnilAndheri, Mumbai, MaharashtraJoyChandni Chowk, DelhiRonBandra, Mumbai, MaharashtraReenaOld Airport Road, Bengaluru, KarnatakaNow, if you want to just extract the information of users who stay in Mumbai, you can do that using the LIKE command and the % operator.SELECT * from user_info where address LIKE '%Mumbai%'The output will benameaddressAnilAndheri, Mumbai, MaharashtraRonBandra, Mumbai, MaharashtraNotice that we have added % operator on both sides of Mumbai. This means that anything can precede Mumbai and anything can be after Mumbai. We just want the string to ...
Read MoreHow to apply DISTINCT constraint on select columns in queries in PostgreSQL?
Suppose you have a table exam_scores containing 5 columns. An example is given below with some dummy data.nameroll_nosubjecttotal_marksmarks_obtainedAnil1English10056Anil1Math10065Anil1Science10045Roy2English10078Roy2Math10098Roy2Science10067Now, one student could have sat for exams of multiple subjects, and therefore, there are multiple rows for 1 student. If you wish to find out the total number of students in the class, you may want to find the number of distinct values of roll_no. You can apply the distinct constraint on a specific column as follows −SELECT DISTINCT ON (roll_no) name, roll_no FROM exam_scores ORDER BY roll_no DESCHere’s what the output of the above query will look like −nameroll_noRoy2Anil1You can also ...
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