有没有办法限制受UPDATE语句影响的行数？

Question

just wondering if there is any way for me to limit the amount of rows which will be affected by an UPDATE statement....I was thinking of using something like a LIMIT statement but there's no such thing in PERVASIVE, any tips?

thank you

Answer 1

It seems that this would work...

UPDATE TableName SET ColumnName = value WHERE ID IN (SELECT TOP 100 ID FROM TableName WHERE CONDITION)

Make TOP 100 whatever your "limit" is. Then, just update the WHERE clause appropriately.

Answer 2

The TOP 100 idea can work but depending on your data, it may skew either testing the correctness or performance. It can also be hard to write. I think adding a RANDOM -based clause works a little better.

UPDATE t SET c = blah 
  WHERE (/* whatever bunch of stuff */) 
  AND Random()<0.01 /* 1 percent */

This works if your DB's random gives a value between zero and one, and can be modified for different random functions and densities.

Answer 3

If you're just being careful, why not just count the rows you would update?

select count(*) from MYTABLE WHERE ...some condition...;

If the count is sufficiently low, this is practically cut & paste to the update:

update MYTABLE set col1 = val1, ... WHERE ...some condition...;

Put into your scripting language, pl/sql, or stored procedure as is appropriate.