MySQL - Rows to Columns

I'm going to add a somewhat longer and more detailed explanation of the steps to take to solve this problem. I apologize if it's too long.

I'll start out with the base you've given and use it to define a couple of terms that I'll use for the rest of this post. This will be the base table:

select * from history;

+--------+----------+-----------+
| hostid | itemname | itemvalue |
+--------+----------+-----------+
|      1 | A        |        10 |
|      1 | B        |         3 |
|      2 | A        |         9 |
|      2 | C        |        40 |
+--------+----------+-----------+

This will be our goal, the pretty pivot table:

select * from history_itemvalue_pivot;

+--------+------+------+------+
| hostid | A    | B    | C    |
+--------+------+------+------+
|      1 |   10 |    3 |    0 |
|      2 |    9 |    0 |   40 |
+--------+------+------+------+

Values in the history.hostid column will become y-values in the pivot table. Values in the history.itemname column will become x-values (for obvious reasons).

When I have to solve the problem of creating a pivot table, I tackle it using a three-step process (with an optional fourth step):

1. select the columns of interest, i.e. y-values and x-values
2. extend the base table with extra columns -- one for each x-value
3. group and aggregate the extended table -- one group for each y-value
4. (optional) prettify the aggregated table

Let's apply these steps to your problem and see what we get:

Step 1: select columns of interest. In the desired result, hostid provides the y-values and itemname provides the x-values.

Step 2: extend the base table with extra columns. We typically need one column per x-value. Recall that our x-value column is itemname:

create view history_extended as (
  select
    history.*,
    case when itemname = "A" then itemvalue end as A,
    case when itemname = "B" then itemvalue end as B,
    case when itemname = "C" then itemvalue end as C
  from history
);

select * from history_extended;

+--------+----------+-----------+------+------+------+
| hostid | itemname | itemvalue | A    | B    | C    |
+--------+----------+-----------+------+------+------+
|      1 | A        |        10 |   10 | NULL | NULL |
|      1 | B        |         3 | NULL |    3 | NULL |
|      2 | A        |         9 |    9 | NULL | NULL |
|      2 | C        |        40 | NULL | NULL |   40 |
+--------+----------+-----------+------+------+------+

Note that we didn't change the number of rows -- we just added extra columns. Also note the pattern of NULLs -- a row with itemname = "A" has a non-null value for new column A, and null values for the other new columns.

Step 3: group and aggregate the extended table. We need to group by hostid, since it provides the y-values:

create view history_itemvalue_pivot as (
  select
    hostid,
    sum(A) as A,
    sum(B) as B,
    sum(C) as C
  from history_extended
  group by hostid
);

select * from history_itemvalue_pivot;

+--------+------+------+------+
| hostid | A    | B    | C    |
+--------+------+------+------+
|      1 |   10 |    3 | NULL |
|      2 |    9 | NULL |   40 |
+--------+------+------+------+

(Note that we now have one row per y-value.) Okay, we're almost there! We just need to get rid of those ugly NULLs.

Step 4: prettify. We're just going to replace any null values with zeroes so the result set is nicer to look at:

create view history_itemvalue_pivot_pretty as (
  select 
    hostid, 
    coalesce(A, 0) as A, 
    coalesce(B, 0) as B, 
    coalesce(C, 0) as C 
  from history_itemvalue_pivot 
);

select * from history_itemvalue_pivot_pretty;

+--------+------+------+------+
| hostid | A    | B    | C    |
+--------+------+------+------+
|      1 |   10 |    3 |    0 |
|      2 |    9 |    0 |   40 |
+--------+------+------+------+

And we're done -- we've built a nice, pretty pivot table using MySQL.

Considerations when applying this procedure:

• what value to use in the extra columns. I used itemvalue in this example
• what "neutral" value to use in the extra columns. I used NULL, but it could also be 0 or "", depending on your exact situation
• what aggregate function to use when grouping. I used sum, but count and max are also often used (max is often used when building one-row "objects" that had been spread across many rows)
• using multiple columns for y-values. This solution isn't limited to using a single column for the y-values -- just plug the extra columns into the group by clause (and don't forget to select them)

Known limitations:

• this solution doesn't allow n columns in the pivot table -- each pivot column needs to be manually added when extending the base table. So for 5 or 10 x-values, this solution is nice. For 100, not so nice. There are some solutions with stored procedures generating a query, but they're ugly and difficult to get right. I currently don't know of a good way to solve this problem when the pivot table needs to have lots of columns.

SELECT 
    hostid, 
    sum( if( itemname = 'A', itemvalue, 0 ) ) AS A,  
    sum( if( itemname = 'B', itemvalue, 0 ) ) AS B, 
    sum( if( itemname = 'C', itemvalue, 0 ) ) AS C 
FROM 
    bob 
GROUP BY 
    hostid;

If you could use MariaDB there is a very very easy solution.

Since MariaDB-10.02 there has been added a new storage engine called CONNECT that can help us to convert the results of another query or table into a pivot table, just like what you want: You can have a look at the docs.

First of all install the connect storage engine.

Now the pivot column of our table is itemname and the data for each item is located in itemvalue column, so we can have the result pivot table using this query:

create table pivot_table
engine=connect table_type=pivot tabname=history
option_list='PivotCol=itemname,FncCol=itemvalue';

Now we can select what we want from the pivot_table:

select * from pivot_table

More details here

You can use a couple of LEFT JOINs. Kindly use this code

SELECT t.hostid,
       COALESCE(t1.itemvalue, 0) A,
       COALESCE(t2.itemvalue, 0) B,
       COALESCE(t3.itemvalue, 0) C 
FROM history t 
LEFT JOIN history t1 
    ON t1.hostid = t.hostid 
    AND t1.itemname = 'A' 
LEFT JOIN history t2 
    ON t2.hostid = t.hostid 
    AND t2.itemname = 'B' 
LEFT JOIN history t3 
    ON t3.hostid = t.hostid 
    AND t3.itemname = 'C' 
GROUP BY t.hostid

I'm sorry to say this and maybe I'm not solving your problem exactly but PostgreSQL is 10 years older than MySQL and is extremely advanced compared to MySQL and there's many ways to achieve this easily. Install PostgreSQL and execute this query

CREATE EXTENSION tablefunc;

then voila! And here's extensive documentation: PostgreSQL: Documentation: 9.1: tablefunc or this query

CREATE EXTENSION hstore;

then again voila! PostgreSQL: Documentation: 9.0: hstore