SQL query results can be ordered by particular attributes. □ Two main categories of query results: □ “Not ordered by anything”. ▫ Tuples can appear in any. Background. SQL, Structured Query Language, is a programming language designed to manage data stored in relational databases. SQL operates through. •Differentiate between SQL statements and SQL*Plus commands. Lesson Aim. To extract data from the database. you need to use the structured query language .
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This is perhaps the most widely used of SQL queries examples. In the example below, we are extracting the “Student_ID” column or attribute from the table. BASIC SQL. ▫ Structured Query Language. ▫ Considered one of the major reasons for the commercial success of relational databases. ▫ Statements for data . SQL stands for Structured Query Language. This tutorial will give you a quick start to SQL. It covers most of the topics required for a basic understanding of SQL.
If you selected the wrong table or changed your mind, the X button here could be used to remove it from the list. Middle and right parts of Query Generator form After the table selection, you would see that the middle part of the form shows you the fields from the highlighted table in alphabetical order. The right part shows the query component when you double-click the field name or table names on a proper box. Under Select, three fields are selected from two tables. Under From, the table names are automatically shown with the default alias T0 and T1.
The default link by system is also shown. If the link is not correct, you can manually fix it.
Under Where, you can choose any fields to restrict the query result. Here, T1. This appears after clicking on the Conditions button.
You may find 12 conditional formulas from the form that can be selected. It can allow the user to select or input values during query execution. Under Sort, T1. Executing a query from Query Generator form When all the required information has been selected, click on Execute.
Then the following form Query—Selection Criteria will pop up for you to input any letters: In the previous example, a letter Y has been entered. It might be saved under any query categories with the name you entered.
The topic regarding query saving will be discussed in detail later. The Reverse Table button at the bottom of the form is used to help you choose to display the table either from right-to-left or from left-to-right.
This is because unlike English, some of the other languages may not start from left-to-right, but in reverse order. You may notice that all query script from different sections of the generator has been linked together for you. Remember, you do not need to write any single statement. This is such a good gift for you to reduce your learning curve in terms of query learning.
Do not waste this valuable resource! Query Wizard Query Wizard is the second tool to be discussed in this chapter. It is similar to Query Generator. We are going to compare both tools later in the book.
Query Wizard overview Query Wizard enables easy access to the database and an easy way of building userdefined reports. This menu item can be accessed from Tools Queries Query Wizard. After you click on Query Wizard menu item, you get the first screen that simply tells you: This wizard will guide you step-bystep through the definition of parameters required for a query.
The screenshot is omitted here since it is nothing but a splash screen for you to know you are starting this wizard. Step 2—Select tables for the report The second step is similar to the left part in Query Generator. You can select as many tables as you need. However, you must try to minimize the number of tables for system performance and query efficiency.
Each table selected is placed in a separate row in the window. The second column displays the full description of the tables. One thing here is, it is noticeably better than Query Generator.
When you select any table, it automatically shows all linked tables under the lower part of the form. You will then find it very convenient to just choose the necessary tables by double-clicking.
This process applies to all tables selected in the upper part of the form.
A linked table list in the lower part of the form changes when you highlight different tables from the upper part of the form. Step 3—Select fields and sort orders Step 3 in Query Wizard has the same function as the middle part of Query Generator. In addition, you have more options to select fields. It is just like when you selected tables from both tools. You may type in any letters on the Find textbox to search your requested fields.
If that is not available to you, this might be the second best place to find all commonly used field information. A subquery is called inner query which returns output that is to be used by another query. Question 24 How many row comparison operators are used while working with a subquery? Question 25 What is the difference between clustered and non-clustered indexes? One table can have only one clustered index but multiple nonclustered indexes.
Clustered indexes can be read rapidly rather than non-clustered indexes. Question 28 How to write a query to show the details of a student from Students table whose name starts with K?
Subquery within another subquery is called as Nested Subquery. If the output of a subquery is depending on column values of the parent query table then the query is called Correlated Subquery.
Question 30 What is Normalization? Here are a few of the most important ones — each of these queries is consequential to almost every system that interacts with an SQL database. This is perhaps the most widely used of SQL queries examples. If you want to display all the attributes from a particular table, this is the right query to use:.
There are a lot of built-in math functions like COUNT and AVG which provide basic functionalities of counting the number of results and averaging them respectively. This query displays the total number of customers by counting each customer ID. In addition, it groups the results according to the country of each customer. A view is a tailored table that is formed as a result of a query. It has tables and rows just like any other table.
A user-defined table is a representation of defined information in a table, and they can be used as arguments for procedures or user-defined functions. A primary key uniquely identifies all values within a table. Foreign keys link one table to another — they are attributes in one table which refer to the primary key of another table. The list of defined triggers can be viewed using the following query.
Internal tables are formed as a by-product of a user-action and are usually not accessible. The data in internal tables cannot be manipulated; however, the metadata of the internal tables can be viewed using the following query. A stored procedure is a group of SQL queries that logically form a single unit and perform a particular task. Thus, using the following query you can keep track of them:.
In this and subsequent examples, we will use a common company database including several tables which are easily visualized. Our practice DB will include a Customers table and an Order table. With this in mind, we can easily imagine an Orders table which likewise contains the indexed customer ID field, along with details of each order placed by the customer.
In our first one of SQL examples , imagine a situation where the zip and phone fields were transposed and all the phone numbers were erroneously entered into the zip code field. We can easily fix this problem with the following SQL statement:. Now, suppose that our data entry operator added the same Customers to the Customers table more than once by mistake.
As you know, proper indexing requires that the key field contain only unique values. Next, imagine that our Customers table has grown to include thousands of records, but we just want to show a sample of 25 of these records to demonstrate the column headings and The SELECT TOP clause allows us to specify the number of records to return, like a Top list.
In this example we will return the top 25 from our Customers table:. Today is Wednesday, and we arrive at work and discover that our new data entry clerk in training has entered all new orders incorrectly on Monday and Tuesday. We wish to teach our new trainee to find and correct all erroneous records.
The Between clause makes the task a breeze:. Undoubtedly the whole reason that a relational database exists in the first place is to find matching records in two tables!