Before delving into identifying and creating keys, it is essential to have a clear understanding of what keys are in the world of Data Analysis. A key is a unique identifier for a row within a database table, and it helps in the creation of relations between different tables. They come in two types – Primary Keys and Foreign Keys.
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Primary Key
: A primary key is a unique identifier for each row in a table. It cannot contain NULL values and it must contain unique values. There can only be one primary key in a single table.
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Foreign Key
: A foreign key is a field or collection of fields in a table that is Primary key in another table. It can accept multiple null value and duplicate value. Foreign Key in one table points to the Primary Key in another table.
Table: Primary Key vs Foreign Key
| Primary Key | Foreign Key | |
| Unique Identifier | Yes | Not necessarily |
| Can be Null | No | Yes |
| How many per table | One | Can be more than one |
Identifying Keys
In order to identify keys in a given table:
- The primary key must be a column (or a set of columns) that uniquely identifies each record in the table. To identify a primary key, one must look for columns that hold unique values for each record.
- The foreign key will be a column that holds values that correspond to a primary key in another table.
Creating Keys in Power BI
After identification, the next step is the creation of keys in Power BI. It can be done using Power Query.
- Load your dataset into Power BI Desktop.
- Go to ‘Edit Queries’ to open the Power Query Editor.
- Select the column you want to convert into a key.
- On the ‘Add Column’ tab, click on ‘Index Column’.
- Choose ‘From 1’ for the index to start from 1. Click ‘OK’.
- Rename the new column as required.
- Click ‘Close and Apply’ to apply the changes.
The added Index Column serves as a unique primary key for the table, and it can be used to create relationships with other tables.
In conclusion, understanding and creating keys in Power BI forms the basis of managing and identifying relationships between different tables in a database for effective data analysis. This skill is without a doubt, a must-have for anyone preparing for the PL-300 Microsoft Power BI Data Analyst exam.
Remember, consistent and diligent practice is key (no pun intended) for gaining proficiency in these areas. So, keep practicing and understanding these principles until you are comfortable with them. Happy studying!
Practice Test
True or False: Primary keys are used to ensure data in the specific column is accurate, reliable, and able to uniquely identify all row data within a table.
- Answer: True
Explanation: A primary key can uniquely identify each record in a table and helps maintain integrity by making sure the column contains unique, non-null values.
True or False: Foreign keys are used to prevent actions that would destroy links between tables.
- Answer: True
Explanation: A foreign key maintains referential integrity by ensuring the action does not cause duplicate data, missing data, or inconsistencies.
Multiple select: In Power BI, which type of keys can be used to create relationships?
- a) Primary Key
- b) Foreign Key
- c) Composite Key
- d) Unique Key
Answer: a, b, c
Explanation: Power BI can use Primary Key, Foreign Key and Composite Key to join tables and establish relationships.
Single Select: Which one of these keys allows null values?
- a) Primary Key
- b) Composite Key
- c) Foreign Key
- d) None of the above
Answer: c) Foreign Key
Explanation: Unlike primary keys, foreign keys can allow null values.
True or False: A foreign key in one table points to a unique key in another table.
- Answer: True
Explanation: A foreign key is a column or a set of columns in one table that refers to the primary key in another table.
Single Select: Composite key is a combination of__________.
- a) Primary Key and Unique Key
- b) Primary Key and Foreign Key
- c) Foreign Key and unique key
- d) None of the above
Answer: b) Primary Key and Foreign Key
Explanation: Composite key is a combination of two or more columns (Primary Key and Foreign Key) used to create a unique identifier for the records.
True or False: In Power BI, it’s okay for a column containing unique key to have null values.
- Answer: False
Explanation: Unique keys need to have a unique value for each row of data and null values compromise this uniqueness.
Multiple select: Which keys can be used as a Primary key?
- a) Composite Key
- b) Foreign Key
- c) Unique Key
- d) None of the above
Answer: a, c
Explanation: Both Unique Key and Composite Key can act as a primary key in a table to ensure uniqueness and integrity.
True or False: A table can have more than one primary key.
- Answer: False
Explanation: There can only be one primary key in a table. However, the primary key can consist of one or more individual columns.
Single select: Power BI relationships require that matching columns in both tables__________.
- a) Must contain unique values
- b) Must contain only integer values
- c) Must contain the similar numerical values
- d) None of the above
Answer: a) Must contain unique values
Explanation: To create a relationship in Power BI, matching columns in both tables should contain unique values to maintain data integrity.
Interview Questions
What is a primary key in relational database design?
A primary key is a specific type of database constraint that identifies each record in the database uniquely. It consists of one or more columns.
What is a foreign key?
A foreign key is a field or a collection of fields that is used to establish a link between the data in two tables. It refers to the primary key in another table.
Can a table have multiple foreign keys?
Yes, a table can have multiple foreign keys depending on its relationships with other tables.
What is meant by composite key?
A composite key is a combination of two or more columns in a table that can be used to uniquely identify each row in the table when the columns are combined.
In what scenario would you use a composite key?
You would use a composite key when a primary key is not sufficient to uniquely identify records. It is helpful in creating complex relationships and ensuring data integrity.
What is the main difference between a primary key and a composite key?
A primary key is a single column that uniquely identifies a record in a table whereas a composite key is a set of columns used together to identify a record uniquely.
How does a foreign key ensure referential integrity?
A foreign key ensures referential integrity by restricting the actions that would violate links between tables. A record in one table cannot be deleted or changed if it is referenced by a foreign key in another table.
What is a surrogate key in database design?
A surrogate key is a substitution for the natural primary key. It is a unique identifier, often a number, for either an entity in the modeled world or an object in the database.
What is the role of a primary key in a many-to-many relationship?
In a many-to-many relationship, a join table is used where the primary key is a combination of the foreign keys.
Can a primary key be null or duplicate?
No, a primary key cannot be null or duplicate. Each value must be unique and exist for each record.
Can a foreign key exist without a corresponding primary key?
No, a foreign key cannot exist without a corresponding primary key. It should always reference a valid and existing primary key.
How do you create a relationship using keys in Power BI?
In Power BI, relationships are created by specifying a primary key column in one table and a foreign key column in another. These relationships allow Power BI to cross-filter and relate data.
What does it mean for two keys to have a one-to-one relationship?
A one-to-one relationship means that each unique value in the first key matches with a unique value in the second key. For each row in the first key, there is only one corresponding row in the second key.
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