Is it possible to set the standard value of a column to the value of another column?

Question

I want the standard value of column B to be the value of Column A in the same row until it is changed through a user triggered UPDATE. Is that possible?

I'm using Mysql 8 and phpmyadmin, if that matters.

Answer 1

MySQL does not allow you to default a value using an expression with other columns.

One solution is an insert trigger on the table to set B to A. However, if A changes, this doesn't keep track -- unless you have an update trigger. But then you don't know when B is set manually or not. Oh, this is hard to keep track of.

You can do something else using a computed column -- assuming that you never want B to be set manually to NULL. This looks like:

create table t as (
     . . . ,
     a int not null,
     _b int,
     b int generated always as (coalesce(_b, a))
);

Then, in your code, you use b for querying. When you want to set the value, you would set the value of _b.

Of course, you could extend this to have a boolean flag on whether a or _b is used for b. However, using NULL to distinguish whether a value is set illustrates the idea and is simpler.

Answer 2

Please see the answer by John C in MySQL: set field default value to other column

I was able to alter a table setting the default value as the concatenation of two other columns in that table:

ALTER TABLE t_table MODIFY IDOne_IDTwo varchar(12) DEFAULT ( concat(IDOne,'-',IDTwo) );

Be sure to wrap parentheses around your expressions and leave the literals bare. MySQL documentation regarding allowing expressions as default values: Explicit Default Handling as of MySQL 8.0.13.

Answer 3

The cache tag bits are the bits within an address (from the perspective of the CPU) that are used as a tag based on the size and width of the cache.

Let us assume a very simple cache with 8 64 byte lines

the 6 least significant bits represent a location within a 64 byte line. The next 3 bits would be the tag, since we only have 8 lines

bits in address: ... xxxx xxxt ttxx xxxx

Addresses 0x86 and 0x10080 would have the same tag in this example

This is an oversimplified example, and there are many nuances to caches, so I would recommend reading some more in depth material on the topic, or read about an actual implementation (i.e. a CPU manual) to get a much better feel for how this works