mvzink commented on code in PR #1757:
URL:
https://github.com/apache/datafusion-sqlparser-rs/pull/1757#discussion_r1986222154
##########
src/ast/mod.rs:
##########
@@ -2947,6 +2947,17 @@ pub enum Statement {
variables: OneOrManyWithParens<ObjectName>,
value: Vec<Expr>,
},
+
+ /// ```sql
+ /// SET <variable> = expression [, <variable> = expression]*;
+ /// ```
+ ///
+ /// Note: this is a MySQL-specific statement.
+ /// Refer to [`Dialect.supports_comma_separated_set_assignments`]
+ SetVariables {
+ variables: Vec<ObjectName>,
+ values: Vec<Expr>,
+ },
Review Comment:
It seems @MohamedAbdeen21 was right that this aspect should be a different
PR to have a more comprehensive discussion. I suggested this new variant in
[another
comment](https://github.com/apache/datafusion-sqlparser-rs/pull/1757#discussion_r1984144488)
because I think that this is a different enough syntactic construct from the
other two existing styles (single-variable assignment a la Postgres and
parenthesized multiple-variable assignment a la Snowflake) that adding another
flag to differentiate them would be unclear and cumbersome. Already, the use of
`OneOrManyWithParens` to differentiate the Postgres and Snowflake styles is, in
my opinion, inferior to a higher level enum variant differeniating the two
styles. Furthermore, especially in the case of the new MySQL-style
multiple-assignments-of-single-variables, I don't think that there enough in
common or a way in which dialects would need elements from both, that warrants
having them in the same variant.
However, I think my opinion would be clearer with two changes. First, when I
initially suggested adding a new variant, I was imagining something more
similar to `Case` with its `Vec<CaseWhen>`: i.e. having a
`Vec<VariableAssignment>` or `Vec<(ObjectName, Expr)>` instead of two vecs
which you later have to `zip` together to do anything with (i.e. in the
`Display` implementation). This loses the structure that was parsed and only
makes sense for the Snowflake style. Again, having that syle also be used for
the Postgres single-assignment style is cumbersome and unclear, and this is
similar. I actually didn't notice this difference from my original suggestion
when re-reviewing this PR or I would have raised it.
The second change that might make my thinking clearer is the other one I
suggested in that comment, which is to unify some or all `SET` statements in
another struct/enum. I originally suggested `Statement::Set(SetStatement)` to
include `SetNames` etc., but I actually think
`Statement::SetVariable(SetVariable)` only covering these 3 cases would be
better. Then any common handling between different types of variable
assignments could still be handled centrally, but the 3 distinct styles could
be represented more concretely, similar to this:
```rust
enum SetVariable {
/// SQL Standard-style
/// SET a = 1;
SingleAssignment {
local: bool,
hivevar: bool,
variable: ObjectName,
value: Expr,
}
/// Snowflake-style
/// SET (a, b) = (1, 2);
ParenthesizedAssignments {
variables: Vec<ObjectName>,
values: Vec<Expr>,
}
/// MySQL-style
/// SET a = 1, b = 2;
MultipleAssignments {
assignments: Vec<(ObjectName, Expr)>
}
}
```
Shoe-horning all 3 of these into one struct/variant via a combination of
`OneOrManyWithParens` and a new boolean flag differentiating Snowflake-style
from MySQL-style that only applies when `variables` matches
`OneOrManyWithParens::Many(_))` seems really annoying without, as far as I can
tell, buying us anything.
If y'all think it would be clear enough to do it that way, I won't stand in
the way, but I really think some kind of new variant/structure (if not
precisely this one) would be a huge improvement.
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