* SQL Expressions: Add Null-literal node
* Retain some order in the code - put NullVal with BoolVal
* Add support for `IN` keyword
* Add GROUP_CONCAT keyword
* Add COLLATE keyword
From Claude:
The test case demonstrates a simple use of COLLATE with a string
literal, but in real MySQL queries, COLLATE is often used in more
complex scenarios like:
1. String comparisons:
`SELECT * FROM table WHERE name COLLATE utf8mb4_bin = 'John'`
2. Sorting:
`SELECT * FROM table ORDER BY name COLLATE utf8mb4_unicode_ci`
3. JOINs:
`SELECT * FROM table1 JOIN table2 ON table1.name COLLATE utf8mb4_bin = table2.name`
The COLLATE clause is particularly useful when you need case-sensitive
comparisons (utf8mb4_bin) or specific language-aware sorting rules.
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