Greg,

> You lost me on that one.  What's a "vertical child table"?

Currently, you store data like this:

id      address uptime  speed   memory  tty
3       67.92   0.3             11.2            37              6
7       69.5            1.1             NULL    15              NULL
9       65.5            0.1             NULL    94              2

The most efficient way for you to store data would be like this:

main table
id      address
3       67.92
7       69.5
9       65.5

child table
id      value_type      value
3       uptime          0.3
3       speed           11.2
3       memory          37
3       tty                     6
7       uptime          1.1
7       memory          15
9       uptime          0.1
9       memory          94
9       tty                     2

As you can see, the NULLs are not stored, making this system much more 
efficient on storage space.

Tommorrow I'll (hopefully) write up how to query this for comparisons.   It 
would help if you gave a little more details about what specific comparison 
you're doing, e.g. between tables or table to value, comparing just the last 
value or all rows, etc.

-- 
-Josh Berkus
 Aglio Database Solutions
 San Francisco


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