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Database Management - Using Full Joins in SQL

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Database Management - Using Full Joins in SQL

  1. 1. FULL JOIN Brought to you by… The Four Marketeers
  2. 2. Why use full join? Glad you asked. The Full Join function combines records from two tables into one display, automatically inserting “null” values for missing matches on either side Full Join can be used to combine all columns from two tables into one comprehensive table, OR you can choose specific columns from each table to combine information
  3. 3. How do you Full Join? First, you select the column from the table you would like to join with another TABLE (use the * function if you want all of the columns to display) Once you do a SELECT statement clarifying this, you FULL JOIN with the target table Then, you select which common column, or field, you would like to initiate the full join with. Ta-Da! Your full join is complete. Let’s look at some examples!
  4. 4. Syntax SELECT * FROM table1 -> FULL JOIN table2 -> USING (common column);
  5. 5. This join is between the tables “books” and “borrowers” using the common column “book_id”
  6. 6. Joined Table Display In this example, using the common column “book_id”, we created a FULL JOIN between books and borrower tables. All the columns in both tables now display together.
  7. 7. This join is between “patrons” and “borrowers” using the common column “patron_id”
  8. 8. A Closer Look
  9. 9. NULL Values Where there is no overlapping information, a NULL will appear in the row; thus, you can easily identify where there is nothing populated in a field
  10. 10. When is using Full Joins appropriate? Examples using our in class library: ● To see if a title has circulated recently ● To see what patrons have borrowed ● To see if any items have not been returned The possibilities are endless...
  11. 11. Other Ways to use FULL JOIN ● In some situations you might want to merge information contained in one table with information in another table. This would display the same information, but differently. ● For instance, to display the descriptive information for an ID in a junction table, you can create a FULL JOIN between the column containing the text value of an ID in a table that otherwise wouldn’t contain it. This is hard to explain with words. Lets look at the next slide and revisit this information on the other end.
  12. 12. So…. ● If you want to see the titles of books in the borrower table, rather than the book ID, you can do a FULL JOIN to display this information. ● This would work for any field where an ID is displayed. You can basically use the FULL JOIN to connect any information contained in one column in one table, to information contained in a separate table. ● To summarize, a FULL JOIN combines all of the effects of applying left and right joins
  13. 13. Later Nerds