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Understanding NULL

From: MySQL Essential Training

Video: Understanding NULL

Null is a special state for the result no value. I noticed in the results in the a column the world null. Null is this special state in SQL.

Understanding NULL

Null is a special state for the result no value. This is necessary because it's often important to distinguish between zero value or an empty string for a nonvalue result. For this lesson we're going to use the scratch database and I'm going to create a simple table here, and I'm going to insert a row. And we'll go and display that row with select. I noticed in the results in the a column the world null. Null is this special state in SQL. It's not really a value.

It's the lack of a value. So you cannot test for it as you would a normal value with the equal sign because it has no value. So if I say, SELECT ASTERISK FROM test WHERE a equals NULL, we'll get no result because there is no row where a is equal to NULL, because NULL is not a value so it cannot satisfy the equal to expression. NULL is a lack of value, so to test for NULL you say a is null instead of equal to null.

So if I test for where a is null, I get that row "Is Null" is a special condition in SQL just like "IS NOT NULL" is a special condition. Of course we get no result there because there aren't any rows in this table. Where it a is not null. So, just to make sure that we've got the distinction clear, I'm going to delete this row and I'm going to insert a new row. And, we'll give it values of zero.

Null and an empty string. Now, if I say select from test where b is Null, we get that row because b is Null in that row. But if I say select from test where b is equal to an empty string, I get no rows. Or if I test for where c equals null or c is null, I get no rows. If I check for where c is not null, then I get a row.

So, it's important to understand that null is not a value. Null is a state that represents a lack of value. Understanding this and how it works is an important part of writing good, robust SQL. Now, I'm going to drop this table So that the database is back in its original state for the next chapter.

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This video is part of

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MySQL Essential Training

60 video lessons · 4664 viewers

Bill Weinman
Author

 
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  1. 4m 22s
    1. Welcome
      1m 3s
    2. Using the exercise files
      1m 31s
    3. What is MySQL?
      1m 48s
  2. 45m 37s
    1. Installation overview
      3m 16s
    2. Installing XAMPP on Windows
      5m 55s
    3. Installing XAMPP on the Mac
      6m 38s
    4. Setting up MySQL users
      11m 31s
    5. Installing SID on Windows
      5m 43s
    6. Installing SID on the Mac
      6m 6s
    7. Installing time zone support in MySQL on Windows
      6m 28s
  3. 45m 43s
    1. The SELECT statement
      3m 57s
    2. Selecting rows
      4m 57s
    3. Selecting columns
      3m 8s
    4. Sorting results with ORDER BY
      2m 58s
    5. Filtering results with WHERE
      3m 52s
    6. Filtering results with LIKE and IN
      3m 41s
    7. Filtering results with regular expressions
      8m 21s
    8. Inserting rows
      4m 9s
    9. Updating rows
      2m 21s
    10. Deleting rows
      2m 25s
    11. Literal strings
      3m 12s
    12. Understanding NULL
      2m 42s
  4. 41m 47s
    1. Creating a database
      4m 30s
    2. Creating a table
      7m 18s
    3. Creating indexes
      6m 8s
    4. Controlling column behavior with constraints
      4m 46s
    5. Creating an ID column
      6m 58s
    6. Using foreign key constraints
      7m 58s
    7. Altering a table
      4m 9s
  5. 28m 56s
    1. What are data types?
      4m 1s
    2. Numeric types
      5m 21s
    3. String types
      2m 58s
    4. Date and time types
      7m 2s
    5. Bit type
      2m 26s
    6. Boolean values
      2m 15s
    7. Enumeration types
      4m 53s
  6. 32m 34s
    1. String functions
      6m 57s
    2. Numeric functions
      6m 2s
    3. Date and time functions
      4m 12s
    4. Time zones in MySQL
      3m 37s
    5. Formatting dates
      1m 51s
    6. Aggregate functions
      5m 45s
    7. Flow control with CASE
      4m 10s
  7. 7m 6s
    1. Maintaining database integrity with transactions
      4m 46s
    2. Using transactions for performance
      2m 20s
  8. 16m 49s
    1. Updating a table with a trigger
      5m 11s
    2. Preventing automatic updates with a trigger
      7m 29s
    3. Logging transactions with a trigger
      4m 9s
  9. 14m 11s
    1. Creating a simple subselect
      3m 23s
    2. Searching within a result set
      3m 53s
    3. Creating a view
      3m 32s
    4. Creating a joined view
      3m 23s
  10. 12m 26s
    1. Understanding MySQL stored routines
      2m 0s
    2. Creating a stored function
      4m 34s
    3. Creating a stored procedure
      5m 52s
  11. 14m 4s
    1. The multi-platform PDO interface
      3m 44s
    2. Executing the SQL
      4m 8s
    3. Implementing auto-increment IDs
      2m 3s
    4. Using a stored funciton
      4m 9s
  12. 1m 3s
    1. Goodbye
      1m 3s

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