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Databases SQL Basics Finding the Data You Want Filtering Out or Finding Missing Information

Posted by sowmya Badida
sowmya Badida
394 Points

How does SQL know which id to take as we have book_id,patron_id,id in loans table?

select first_name,email from patrons where id =4;

In this query, how does SQL know that it has to select the patron_id only?

4 Answers

Steven Parker
Steven Parker
231,261 Points
Steven Parker
Steven Parker
231,261 Points

The "id" columns all have different names. The column name referenced must match the column name exactly.

The query you listed does not reference "patron_id". It will only look for rows where the column named "id" is equal to 4.

Talha Takleh Omar Takleh
Talha Takleh Omar Takleh
3,082 Points
Talha Takleh Omar Takleh
Talha Takleh Omar Takleh
3,082 Points

Remember at the beginning of this course where the instructor mentioned that database consists of data and schema. Schema is the organisation of data where the data is stored and divided up into different sections. Sections are also known as tables and these tables can relate to one another.

For this example we have a number of tables. Loans, patrons and books. Since we want to find the patron and the relationship between the tables for loans and patrons is the id. If you look at the patrons table, it has column named id. That's why on the query we can just use id but must refer to the patrons table.

Julio Mayorga
Julio Mayorga
1,865 Points
Julio Mayorga
Julio Mayorga
1,865 Points

The above is correct, however, I think he should have looked for id = "5" and not 4. If you look at the table that was pulled it says that the id is 5 and not 4.

Tineyi Chinhengo
Tineyi Chinhengo
30,770 Points
Tineyi Chinhengo
Tineyi Chinhengo
30,770 Points

If you want to select for patrons_id only. SELECT first_name,email FROM patrons WHERE patrons_id ="4";

If you want to select for id only. SELECT first_name,email FROM patrons WHERE id ="4";

Right now you can see the difference between the 2 and you can choose your correct answer.