modify format of multiple database columns at once - mysql

Question

I have imported a csv table into sql db using phpmyadmin. I guess the default format is decimal(8,5), or at least that is how it comes out. Seems overkill and I thought I could reduce to 4,1. Problem is there are around 470 fields. I know how to change one at a time but this would take a long time. Is there a faster way?

Answer 1

I only have Sql Server 2008 R2, but it seems the MySQL solution may be similar. I've tried to align it as closely with MySQL as I can, but without a MySQL engine I can't check it ... So for what it's worth, here's my script (tested in Sql Server):
-- I believe this is 'CREATE TEMPORARY TABLE' in MySql
create table #tbl
(
ownerName sysname -- `sysname` is nvarchar(128) the max identifier length
, tableName sysname
, colName sysname
, colType sysname
, colPrecision int
, colScale int
)
insert #tbl
select table_schema, table_name, column_name, data_type, numeric_precision, numeric_scale
FROM information_schema.columns
where data_type = 'decimal' and numeric_precision = 8 and numeric_scale = 5
-- Note: It is 'MODIFY COLUMN' in MySQL
declare #newPrecision int = 4, #newScale int = 1
DECLARE #sql varchar(max) = ''
select #sql = #sql + CHAR(13) + CHAR(10) + 'ALTER TABLE ' + ownerName + '.' + tableName + ' ALTER COLUMN '
+ colName + ' decimal(' + cast(#newPrecision as varchar(max)) + ',' + cast(#newScale as varchar(max)) + ')'
from #tbl
/**
-- In MySql, GROUP_CONCAT() may work
select #sql = GROUP_CONCAT( 'ALTER TABLE ' + ownerName + '.' + tableName + ' MODIFY COLUMN '
+ colName + ' decimal(' + cast(#newPrecision as varchar(max)) + ',' + cast(#newScale as varchar(max)) + ')' SEPARATOR ' ' )
from #tbl
**/
print #sql
execute ( #sql )

Answer 2

I'd never used SqlFiddle before so thought it was a good time to start, to give a complete answer specifically for MySQL 5.6. Please see http://sqlfiddle.com/#!9/19f46/1
At the bottom of Build Schema panel, select Delimiter = [ // ]. SqlFiddle uses this to decide when to send off a chunk of code to MySQL. It is necessary because the CREATE PROCEDURE has to be sent off as one chunk. If you use the default Delimiter = [ ; ], then it will send off only part of the CREATE PROCEDURE up to the first ; it finds.
The table cols selects columns of type decimal with the precision and scale you want to change. Currently hardcoded as 8 and 5 as the OP requested, but change as required to identify only the columns you want to modify. It would be a good idea before you do the table modifications to run this select to verify which columns you will be modifying.
The stored procedure exec_multiple uses the table cols to generate ALTER TABLE statements which are then dynamically executed.
EXECUTE only handles one statement at a time, so you need to iterate through the rows of cols and apply each ALTER TABLE individually. The auto_increment column id in cols allows you to select each row in turn without the use of a cursor.
The table test_log captures any debugging information you may want to examine after the Build Schema is complete.
The following goes in the Build Schema panel on the left.
All of the logic needs to be in this panel because SqlFiddle does not allow Data Definition Language or table insert/update/delete statements in the Run SQL panel.
create table if not exists cols
(
id int auto_increment primary key
, ownerName varchar(128)
, tblName varchar(128)
, colName varchar(128)
, colType varchar(128)
, colPrecision int
, colScale int
) //
create table if not exists test_table
(
testDecimal1 decimal(8,5)
, testDecimal2 decimal(8,5)
) //
create table if not exists test_table2
(
testDecimal3 decimal(8,5)
, testDecimal4 decimal(8,5)
) //
create table if not exists test_log
(
msg varchar(1024)
) //
INSERT INTO cols
( ownerName, tblName, colName, colType, colPrecision, colScale )
SELECT TABLE_SCHEMA, `TABLE_NAME`, COLUMN_NAME, DATA_TYPE, NUMERIC_PRECISION, NUMERIC_SCALE
FROM INFORMATION_SCHEMA.COLUMNS
WHERE DATA_TYPE = 'decimal' AND NUMERIC_PRECISION = 8 AND NUMERIC_SCALE = 5 //
insert test_log( msg) select database() //
CREATE PROCEDURE `exec_multiple` ( newPrecision int, newScale int)
BEGIN
declare n int;
declare nrows int;
declare sql_stmt varchar(1024);
set n = 1;
select count(*) from cols into nrows;
while n <= nrows do
select CONCAT('ALTER TABLE '
, ownerName, '.', tblName, ' MODIFY COLUMN '
, colName, ' decimal(', newPrecision, ','
, newScale, ')') into sql_stmt from `cols` where id = n;
SET #sql_stmt := sql_stmt; -- not sure why this is necessary
insert test_log( msg ) select #sql_stmt;
PREPARE dynamic_statement FROM #sql_stmt;
EXECUTE dynamic_statement;
DEALLOCATE PREPARE dynamic_statement;
set n = n + 1;
end while;
END //
call exec_multiple(4, 1) //
This goes in the Run SQL panel on the right
select * from test_log;
SELECT TABLE_SCHEMA, `TABLE_NAME`, COLUMN_NAME, DATA_TYPE, NUMERIC_PRECISION, NUMERIC_SCALE
FROM INFORMATION_SCHEMA.COLUMNS
WHERE DATA_TYPE = 'decimal'
;
select * from cols;