Implement an in-memory SQL-like table

Problem

Implement a simple in-memory database for one table. All values are strings.

Each row is identified by a rowKey (string). Each row contains columns identified by colKey (string).

You must process a sequence of commands and output results for read/query commands.

Supported commands

1. SET rowKey colKey value
   • Set table[rowKey][colKey] = value .
2. GET rowKey colKey
   • Output the value if present, otherwise output NULL .
3. SELECT whereCol whereValue orderByCol
   • Return all rowKeys for rows where table[rowKey][whereCol] == whereValue .
   • Sort the matching rows by the value of orderByCol in ascending lexicographic order .
   • If a row is missing orderByCol , treat its sort value as an empty string "" .
   • If multiple rows have the same orderByCol value, break ties by rowKey ascending.
   • Output the resulting rowKey s joined by a single space (or output an empty line if none).

Input/Output format

• Input: list of commands (one per line).
• Output:
  • For each GET , print a single line.
  • For each SELECT , print a single line.

Constraints (assume)

• Up to $2 \times 10^5$ commands.
• Total length of all strings is within reasonable memory limits.