根据 2 个条件在 SQL 添加新列

Question

I would like to create a new column in SQL using two conditions. I want to create a column that says 1 if the item count is greater than 1 and if item/sum is greater than 5, otherwise return 0. This is not my original data, is there anyway I could add this new column with these specified rows using a select statement.

Answer 1

First I want to say that using SQL key words or SQL function names as table name or column name should be avoided, so if possible, I recommend to rename the columns "number" and "sum". The second point is it's unclear what should happen in case the sum column is 0. Since a division by zero is not possible, you will need to add a condition for that. Anyway, the way to achieve such things is using CASE WHEN . So let's add the new column:

ALTER TABLE yourtable ADD column column_name INT;

Now, you need to execute an update command for that column providing the logic you want to apply. As an example, you can do this:

UPDATE yourtable SET column_name =
CASE WHEN item <= 1 THEN 0
WHEN sum = 0 THEN 1
WHEN item / sum > 0.5 THEN 1
ELSE 0 END;

This will set the new column to 1 only in case item is > 1 and sum is 0 or sum item / sum is > 0.5 (greater 50%). In all other cases it will be set to 0. Again, the bad column naming can be seen since "WHEN sum..." looks like you really build a sum and not just use a column. If you want as example to set the new column to 0 instead of 1 when the sum is 0, just change it and try out. In case you want to automatically apply this logic fur future inserts or updates, you can add a trigger on your new column. Something like this:

CREATE TRIGGER set_column_name
BEFORE INSERT ON yourtable
FOR EACH ROW
SET new.column_name = CASE WHEN new.item <= 1 THEN 0
WHEN new.sum = 0 THEN 1
WHEN new.item / new.sum > 0.5 THEN 1
ELSE 0 END;

But take care, the syntax of triggers depend on the DB you are using (this example will work in MYSQL). Since you did not tell us which DB you use, you maybe need to modify it. Furthermore, depending on your DB type and your requirements, you need zero, one or two triggers (for updates and for inserts).