2015-01-29 12:34 GMT-02:00 Tom Worster <fsb@xxxxxxxxxx
<mailto:fsb@xxxxxxxxxx>>:
I have one final technical question. It is for my understanding
and does
not reflect a problem I have as a user.
The manual says:
> The answer is: SQL is a statically-typed language. The SQL
interpreter
needs to know the datatypes of all expressions before the query is run
(for example, when one is using prepared statements and runs
"select COLUMN_GET(...)", the prepared statement API requires the
server
to inform the client about the datatype of the column being read
before
the
query is executed and the server can see what datatype the column
actually
has).
This appears to say that the constraint is that the server needs to
determine the type of each column in the data sent to the client.
SQL such
as:
SELECT x, y, COLUMN_GET(stuff, 'z') FROM t
can't work because the datatype of the 3rd column in "table"
returned to
the client can't be determined before reading data.
hum... no
at 3rd column, you have a function returning a value, the function
have a know data type, check this:
http://dev.mysql.com/doc/refman/5.1/en/udf-calling.html
that's how user defined functions are implemented at mysql, think
about dynamic column as many functions to handle a string and
extract/include information about variables with defined data type
for example... when you create a dynamic column with:
-- MariaDB 5.3+:
INSERT INTO tbl SET dyncol_blob=COLUMN_CREATE(1 /*column id*/, "value");
-- MariaDB 10.0.1+:
INSERT INTO tbl SET dyncol_blob=COLUMN_CREATE("column_name", "value");
check: https://mariadb.com/kb/en/mariadb/dynamic-columns/#column_create
you have a STRING as return of function COLUMN_CREATE()
The return value is suitable for
*
o storing in a table
o further modification with other dynamic columns functions
at "value" parameter, COLUMN_CREATE know that we are using a STRING,
but should be any other data type
The *|as type|* part allows one to specify the value type. In most
cases, this is redundant because MariaDB will be able to deduce the
type of the value. Explicit type specification may be needed when the
type of the value is not apparent. For example, a literal
|'2012-12-01'| has a CHAR type by default, one will need to
specify|'2012-12-01' AS DATE| to have it stored as a date. See the
Datatypes
<https://mariadb.com/kb/en/mariadb/dynamic-columns/#Datatypes> section
for further details.
