Good database design suggests that each table lists data only about a single entity , and detailed information can be obtained in a relational database, by using additional tables, and by using a join. A primary key is a column or set of columns that uniquely identifies the rest of the data in any given row.
This means two things: no two rows can have the same OwnerID, and, even if two owners have the same first and last names, the OwnerID column ensures that the two owners will not be confused with each other, because the unique OwnerID column will be used throughout the database to track the owners, rather than the names.
A foreign key is a column in a table where that column is a primary key of another table, which means that any data in a foreign key column must have corresponding data in the other table where that column is the primary key. In DBMS-speak, this correspondence is known as referential integrity.
For example, in the Antiques table, both the BuyerID and SellerID are foreign keys to the primary key of the AntiqueOwners table OwnerID; for purposes of argument, one has to be an Antique Owner before one can buy or sell any items , as, in both tables, the ID rows are used to identify the owners or buyers and sellers, and that the OwnerID is the primary key of the AntiqueOwners table.
The purpose of these keys is so that data can be related across tables, without having to repeat data in every table—this is the power of relational databases. For example, you can find the names of those who bought a chair without having to list the full name of the buyer in the Antiques table…you can get the name by relating those who bought a chair with the names in the AntiqueOwners table through the use of the OwnerID, which relates the data in the two tables.
Note the following about this query…notice that both tables involved in the relation are listed in the FROM clause of the statement. Because the joining condition used an equal sign, this join is called an equijoin.
Dot notation refers to prefixing the table names to column names, to avoid ambiguity, as follows:. This means that you will need to tell SQL to eliminate duplicate sales rows, and just list each person only once. However, keep in mind that since the SellerID column in the Antiques table is a foreign key to the AntiqueOwners table, a seller will only be listed if there is a row in the AntiqueOwners table listing the ID and names.
We also want to eliminate multiple occurrences of the SellerID in our listing, so we use DISTINCT on the column where the repeats may occur however, it is generally not necessary to strictly put the Distinct in front of the column name.
For future reference and in case anyone asks , this type of join is considered to be in the category of inner joins. In this section, we will talk about Aliases , In and the use of sub queries, and how these can be used in a 3-table example. First, look at this query which prints the last name of those owners who have placed an order and what the order is, only listing those orders which can be filled that is, there is a buyer who owns that ordered item :.
This query shows the total of all salaries in the table, and the average salary of all of the entries in the table. This query gives the smallest figure of the Benefits column, of the employees who are Managers, which is What a view does is to allow you to assign the results of a query to a new, personal table, that you can use in other queries, where this new table is given the view name in your FROM clause.
Interviewing for a job as an SQL tester requires a high level of competency in the vast field of data science. While some interview questions may be more common, questions specific to database testing are far more in-depth. For that reason, practice and preparation are necessary to make a good impression on the interviewer. This article aims to provide examples of SQL tester questions and sample answers to help you prepare for your next interview.
Here are some common SQL tester interview questions:. SQL supports software applications by performing tasks such as retrieving, updating, inserting and deleting data. SQL makes it possible to execute queries, insert and update records, create and delete databases and tables, and more. It also acts as a file manager for the data kept in a database, no matter the size or complexity of the database. There are no precise upper limits to the number of records that can be defined by rows in the table.
This is useful to avoid the need to create individual tests for each set of data, which can be a cumbersome process.
The framework keeps the data quarantined, and the same test script can be used to generate results for multiple combinations of input test data. There are four types of data-driven testing:. The two types of DCL commands are:. Grant: grants users access to the database. Connection: forms a connection with a database. My job was to print documents from various combinations of operating systems, browsers, and printers and to verify the print quality of the documents.
For this test, we did not need to apply any SQL skills. SQL experience was required to verify the test data, insert, update, and delete the test data values in the database. While working on a different project, I was involved in back-end testing where complex SQL Query knowledge was a must.
There was an internal user interface tool to get the data from the Oracle database based on the input values. As part of our testing we compared the UI tool output and the database output by inputting the same values to the tool and the database to make sure that the tool was functioning properly.
Every time input values varied, the Database Administrator gave the testing team very big queries with select statement to use but we needed to understand the relation between the tables, columns, and the query before we used it.
In addition, we used different types of SQL Statements to verify the test data. Please reach out to Olenick if you have any questions on SQL, or would like to suggest future topics on this subject!
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