PoStorm - MySQL category

Like column

Mon, 23 Jun 2008 17:17:00 GMT

Like column

Create table t1 ( x int unsigned default 911, y like x )

Create table t2 (x like t1.x)

Tricky extension

Create table t2 (x always like t1.x)

Meaning that

alter table t1 modify x int signed

implies a corresponding alter on t2

The purpose of ‘LIKE’ is to avoid multiply specifying that which must always be the same. LIKE both documents the dependency, and eliminates the possibility of inconsistency. ALWAYS LIKE prevents the inconsistency developing in the future.

ALTER Initial

Sun, 15 Jun 2008 17:14:00 GMT

ALTER Initial

Alter table this,that (ie. ALTER with multiple changes) appears to be optimized perform a single pass thru a table (at least sometimes) to perform the multiple changes. Some changes require column expression evaluation. These could be done with the alter

E.g.

Alter table xxx, add column negative bit initial quantity<0, modify quantity int unsigned initial abs(quantity)

This changes the representation of a column (‘quantity’) from a signed integer to an unsigned integer with a flag (‘negative’) to indicate that quantity is negative.

Alter table sales add unitprice dec(6,2) initial totalprice/quantity , drop totalprice

This alters a table, in a single pass, from recording the quantity and total price of sales, to recording quantity and unit price of sales. Without this extension it requires three passes through the table (1. Alter Add unitprice, 2 update set unitprice, 3 alter drop totalprice).

The meaning of the ‘initial’ clause is an expression that is computed based on the values in the row at the time the ALTER is performed for each row to set (or update) the value of the column.

BitOf(column,string)

Sat, 14 Jun 2008 17:07:00 GMT

BitOf(column,string)

Manipulating Column Type SETs cannot be readily (or efficiently) manipulated

Eg. Suppage the column Flags is defined as set(‘a’,’b’,’c’)

Update flags = flags_with_b_flipped.

This can be written using string manipulation – although it is messy - or can be written as

Flags=flags^2 /* b=~b */

But this depends on ‘b’ being the second item in the set declaration.

Flags=flags^bitOf(flags,’b’)

BitOf(column,string) means the bit-mask that represents string in the column definition. (not the current value of).

Limit [while|until] expression

Fri, 13 Jun 2008 16:51:00 GMT

Limit [while|until] expression

Performing sequential operations on set is a bitch, but this might help.

The MySQL extension ‘limit’ …

Select * from budget_candidates order by priority limit while sum(cost)<1000.00