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.

1. GET rowKey colKey

• Output the value if present, otherwise output NULL.

1. 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 rowKeys 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.