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Flow control with CASE

From: MySQL Essential Training

Video: Flow control with CASE

MySQL supports the standard SQL conditional expression syntax. And so, there's our table.

Flow control with CASE

MySQL supports the standard SQL conditional expression syntax. This is not really a function, but I'm covering it here because it's important to understand it. The syntax is a little cumbersome, but here's how it works. We're going to use the scratch data base for this, and we're going to create a little table. We'll destroy it later. Create table, and we'll call it booltest. Bool for Boolean, and we'll give it two simple columns integers, and we'll insert a couple values into it just a one and a zero we're going to be using these as Boolean values.

In MySQL a one is a Boolean true. And a zero is a Boolean false. Actually, zero is false. And anything else is true. But one is traditionally used for true. And so we'll select from our booltest. So we can see the values here. And so, there's our table. And it has two values in one row, a one and a zero. We're going to use this to test our conditional expression. So, I'm going to go ahead and create a SELECT statement here.

And I'm going to put the body of the SELECT statement. On separate lines, and so there'll be a case here when, and there's actually two different syntax's for case. I'm going to show you one, then I'm going to show you the other one. So when a and it's true if there's a non-zero value in the a column. And it's false if there's a zero value in the a column. THEN, and we'll give the value true, so this returns that value for the select statement.

ELSE false in a string. END this ends the case statement. And so I can say AS boolA, for the next value. And then I'm going to do another case statement. I'm just going to Copy this and Paste it here. We won't need the comma. And we'll select for b and we'll name it boolB. And we'll save FROM booltest. And we have the semicolon on the next line. So what do we expect to have happen? Well.

If a is true then it will return the value true for this column and otherwise it will return the value false. And this will be named a and b will be true or false depending on it b column is true or false. So the a column is true because it has a one and the b column is. False because it has a zero. So, let's see if we get the expected result here boolA is true and boolB is false. So, this is very simply how the case statement works, in this syntax model I'll show you the other one in a moment.

So it has a WHEN clause. And then WHEN clause has a Boolean expression, which in this case is just that column, because it's Boolean true or false. And then an optional ELSE for each WHEN. And then you can have more than one WHEN as well. And END ends the case syntax so that you can go ahead and do whatever else you're going to do with that column as in this case we have an as alias. So the other way this can work is, I can say CASE a and I can say when it's value is one.

Then true ELSE false and likewise with the other column here. I can say b and when one, then true ELSE false. So this allows you to select a particular variable or a column and have multiple possible values for it. So again we get exactly the same result when I press Go. So, now I'm going to remove our temporary table. To return the database to its' original state, and the case expression can be a bit awkward but, it's very useful and you do well to become familiar with it.

Use case wherever you need a conditional in MySQL. You'll see one or two other examples of this later in the course.

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This video is part of

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MySQL Essential Training

60 video lessons · 5563 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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