How to make the query fast of Comparing between different data types IN MYSQL

Question

1) My question is that, when i have two large table which cannot be alter because of there sizes. Now i have to join them on a common field and now compare one field from both the table which is having same data but one's data type is int and anothers is varchar. I know we can done this easily, but when table have millions of record then comparing between two different data type is slow down, how can i make it fast.

2) my similar 2nd question is that when i have to join two tables on some field like id and which is in different data type in both the table. like one is int and another is char.....how can i join this two table because i cannot wait for many days.

(One solution i have tried is to create new table as an abstract(by in file out file) of old . While i have now changed the data type from char to int during create table and then took the in file)

If anybody have any other solution, please share

Answer 1

Make sure the conversion happens on the first table in the join, that way:

1. the conversion only happens only once per row
2. indexes can be used to join with the second table

for example:

select * 
from table1
join table2 on table2.intcol = cast(table1.varcharcol as signed)

This sample query will use an index on table2.intcol (if one exists) to join the two tables.

Answer 2

Yes cast can be used for changing data types

Answer 3

select * 
from table1
join table2 on table2.intcol = cast(col as int)