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Controlling column behavior with constraints

From: MySQL Essential Training

Video: Controlling column behavior with constraints

As you're defining your table you may want to And, then we have the create table, which is two columns, an integer, and a varchar.

Controlling column behavior with constraints

As you're defining your table you may want to define specific rules and behaviors for some of your columns. This is done by using constraints. I'm going to use the Scratch database for this lesson. And I'm going to copy and paste some code here out of the Chapter 3 exercise file, here around line 73. Everything from drop table to select from test. Press Cmd+C to copy, and Cmd+V to paste. That's Ctrl+C and Ctrl+V, if you're using a PC.

And you see, we start here with a drop table. Because, again, we're going to be creating this table over and over again with different options. So we need a drop table at the start of that. And, then we have the create table, which is two columns, an integer, and a varchar. And, we'll insert a row into it, describe it and select from it. And so that looks like this. There's our table and there's our one row. In this example you'll notice that the A column has a null value because no data was added to that column.

If you look here at our insert. We're only inserting data into row B. And so, when no default value is specified, MySQL uses null as a default. If you want to disallow null values, you can add a not null constraint to the column. So that would look like this. So here in our column definition we have the type, and the type is followed by various constraints and in this case we have one constraint which is not null. And you notice in the describe here, it says under null, it says yes.

It'll say no when we redefine the table. And because there's no default value specified, the default value is null, it gives it a zero, because that's the closest that the database engine can do. Other database systems handle that situation differently, but this is what MySQL does with that. When you have not null for an integer, and you do not define a default value. You get a zero. Now you can define a default value right out here in the constraint, so you can say default and give it a value say 47.

And now when I press Go you notice the default here in the describe is set at 47 and our insert which does not specify a value for a. Puts 47 in that row, if I try to specially insert a null. So, I'll say a and b in my insert here and I'll try and put in a null. This will give me a constraint violation error, integrity constraint violation, column a cannot be null. And the insert failed.

So there is no row for the select statement. So we'll go ahead and put that insert back without the value there. And I'm going to show you another constraint. This constraint is called the unique constraint. And if I say unique here, for the b column. And press Go, you'll notice that I now have a key, and it says, unique. And in fact, if I come in here and I say, Show Index From, you'll notice that it's created an index so that it can enforce that uniqueness.

And if I come in here and I try to. Insert the same row a few more times, I'll get an integrity constraint violation, duplicate entry one for key b, because this index is set up as unique. You see non-unique is zero and key is set to unique here and that's because of this unique constraint there in the column definition. Now keep in mind that MYSQL treats null as a special case. It is not a value, and so adding multiple nulls to this column does not count as a integrity constraint violation.

You see we have several nulls there because null is not a value. Null is the absence of value. So if you want to actually prevent that from happening, you can use not null as an additional constraint. And now it says column b cannot be null. And we don't have those nulls at all. So, constraints create rules governing the behaviors of columns in your tables. MySQL supports several different kinds of constraints and we'll explore more of them later in this chapter. For now I'm going to go ahead and drop this table to return our database to its original state for the rest of the course.

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This video is part of

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MySQL Essential Training

60 video lessons · 6604 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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