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Altering a table

From: MySQL Essential Training

Video: Altering a table

Sometimes, you may need to change the definition of a and that'll drop this column, and all the data that's in it.

Altering a table

Sometimes, you may need to change the definition of a table after it's already been defined, and populated with data. You can do this with the alter table statement. We're going to be using the scratch database here for this lesson, and I'm just going to copy a little bit of code out of our exercise file. So this is the chapter 3 exercise file starting around line 201. From the create table to the select from test. Just copy that and paste it into the SQL box here.

So we're creating a simple table with 3 columns and populating some data into them and it looks like that. Now suppose some time is passed, and I decide I want a new column. I can use the alter tables statement like this. I'm just going to leave that select statement there and I'm going to say alter table test add d. And I'll make it the same varchar 10. And when I say go, you see we have a new column d, and it's filled with nulls.

The part after the, so we have alter table test, and the part after the add keyword is really just a normal column definition just like in a create table statement. I can also remove a column with drop. I can say alter table, drop d like this, and that'll drop this column, and all the data that's in it. If I drop a column that has data, that data will be deleted. For example, if I drop column b here, now column b is gone, and all the data is gone with it.

So you have to be very careful removing columns obviously from a live database. As you can see, the data is permanently lost. It cannot be recovered. But you also need to be aware that any code that may depend upon columns that you're removing, that code needs to be modified, make sure that it's not referencing columns that are no longer there. By default, new columns are added after the last column. You saw that our column d ended up there at the end of the table. If I want to, I can add a column in a particular position like this.

I can say alter table, add and I'll call it bb, and it'll be Varchar after a. And we'll have a new column bb after the a column there. And you notice that it nulls that data that was there before, it's gone forever. New columns are filled with null by default, you can specify a different default like this. I can add a new column d and just say default panda like that.

Of course everything here after add is just like a column definition and so when I say go now that d column has panda in it, in fact you can even define auto incrementing columns. I can say, add id serial, and if I want to add a column that's not after anything, that's actually the first column, I can just say first like that. And serial if you remember, is an auto incrementing column. And so it says 1, 2, 3. You can always see the full definition of your table schema with the show create table statement like that.

And that shows you a create table statement that would create exactly this table. And you'll notice there's our big end unsigned not null auto increment for the id column serial. So the alter table command is a powerful tool, and as such, it should be used with care and caution. You need to be careful to track and update any code that may depend on the existing schema before you change it. Sometimes this sort of change is necessary, so it's good to understand how to accomplish it, but extra special care is necessary especially for live databases.

So I'm going to go ahead and drop this table now to return our database to it's original state.

Show transcript

This video is part of

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MySQL Essential Training

60 video lessons · 5859 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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