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https://issues.apache.org/jira/browse/IMPALA-7832?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16737671#comment-16737671
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Paul Rogers commented on IMPALA-7832:
-------------------------------------
Turns out our existing syntax is:
{noformat}
ALTER TABLE <table> (REPLACE|ADD) COLUMNS ( ... )
{noformat}
That is, our syntax allows multiple columns per statement, not just one as in
the ISO SQL. So, we could do two things:
* Add ISO SQL syntax that [~grahn] provided.
* Modify our existing syntax as shown in the original description.
This ensures that we are both SQL-compliant and backward-compatible. I believe
that the two syntax variations can co-exist, I don't believe that they lead to
ambiguities, but the parser generator will tell us if they are.
> Support IF NOT EXISTS in alter table add columns
> ------------------------------------------------
>
> Key: IMPALA-7832
> URL: https://issues.apache.org/jira/browse/IMPALA-7832
> Project: IMPALA
> Issue Type: New Feature
> Components: Frontend
> Affects Versions: Impala 3.1.0
> Reporter: Thomas Tauber-Marshall
> Assignee: Fredy Wijaya
> Priority: Minor
> Labels: ramp-up
>
> alter table <table_name> add [if not exists] columns (<name> <type> [, <name>
> <type>...])
> would add the column only if a column of the same name does not already exist
> Probably worth checking out what other databases do in different situations,
> eg. if the column already exists but with a different type, if "replace" is
> used instead of "add", etc.
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