Sixth Chapter Lesson-4 Database queries and Different operators.

In this Lesson, you will learn-

  • 1.Different database queries.
  • 2.Different database operators.

 

Query: A query is a request for data or information from a database table or combination of tables.

Types of query:

  • 1.Select Query
  • 2.Parameter Query
  • 3.Crosstab Query
  • 4.Action Query

Select Query: Select query is used to retrieve data from a table by selecting fields. It is the most used SQL query.

Parameter Query: A parameter query is one of the simplest and most useful queries that is done by taking input as a parameter from a dialog box, which is then used by the query in an expression or criteria. It is used when you want to create a query that you can use many times, but with a different value each time.

Crosstab Query: A crosstab query summarizes the data from one or more fields that are separated into groups based on one or more fields. Crosstab query is a type of select query. This query calculates a sum, average, or other aggregate function, and then groups the results.

Action Query: An action query is a query that makes changes to or moves many records in just one operation. There are four types of action queries: append, update, make-table, and delete.

  • 1.Append Query– takes the set results of a query and “appends” (or adds) them to an existing table.
  • 2.Delete Query– deletes all records in an underlying table from the set results of a query.
  • 3.Make Table Query– as the name suggests, it creates a table based on the set results of a query.
  • 4.Update Query– allows for one or more field in your table to be updated.

 

Query Language: Query languages are used to make queries in a database. These are some query languages-

  • QUEL (Query Language)
  • QBE (Query By Example)
  • SQL (Structured Query Language)

SQL: SQL stands for Structured Query Language. SQL is a standard language for accessing and manipulating databases. SQL lets you access and manipulate databases. Although SQL is often described as, and to a great extent is, a declarative language (4GL). It also includes procedural elements. SQL consists of many types of statements, which may be informally classed as sublanguages, commonly:

  • 1.Data query language (DQL)
  • 2.Data definition language (DDL)
  • 3.Data control language (DCL)
  • 4.Data manipulation language (DML)

 

Database Operators: An operator is a reserved word or a character used primarily in an SQL statement’s WHERE clause to perform operation(s), such as comparisons and arithmetic operations. Different types of operators-

  • Arithmetic Operators- You use the arithmetic operators to calculate a value from two or more numbers or to change the sign of a number from positive to negative or vice versa.

 

  • Comparison Operators- You use the comparison operators to compare values and return a result that is True, False, or Null.

 

  • Logical Operators- You use the logical operators to combine two Boolean values and return a true, false, or null result. Logical operators are also referred to as Boolean operators.

 

  • Concatenation Operators- You use the concatenation operators to combine two text values into one.

 

  • Special operators- You use the special operators to return a True or False result as described in the following table.

Lesson Evaluation- 

  • a. What is Query?
  • a. What is query language?
  • a. What is SQL?
  • a. What is DDL/DML?

 

  • b. Explain the necessity of queries in database.
  • b. Why SQL is called  the soul of database?
  • b. Why SQL is called very high level language?
  • b. “SQL is a set of some statements” -Explain it.

 

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