Add LIKE operator for pattern matching in WHERE #13

Summary

Add support for the LIKE operator in WHERE clauses for simple pattern matching on string columns.

Expected Behavior

SELECT * FROM 'users.csv' WHERE name LIKE 'John%'
SELECT * FROM 'data.csv' WHERE email LIKE '%@gmail.com'
SELECT * FROM 'products.csv' WHERE sku LIKE 'AB__CD'

Standard SQL LIKE patterns:

• % matches any sequence of characters (including empty)
• _ matches exactly one character

Why This Is a Good First Issue

• Teaches pattern matching algorithm implementation in Zig
• Involves modifying the parser (new operator) and engine (new evaluation logic)
• Self-contained — no dependencies on GROUP BY or other pending features
• A great exercise in recursive or iterative string matching

Suggested Approach

1. Add like to the Operator enum in src/parser.zig
2. Detect LIKE keyword during WHERE clause parsing (between column name and pattern value)
3. Implement a matchLike(text: []const u8, pattern: []const u8) bool function
4. The matching function can be iterative: scan pattern left-to-right, _ consumes one char, % tries matching the rest of pattern against every suffix of text
5. Integrate into WHERE evaluation in src/engine.zig, src/mmap_engine.zig, src/parallel_mmap.zig

Pattern matching pseudocode

fn matchLike(text, pattern) -> bool:
    t = 0, p = 0
    star_p = -1, star_t = -1
    while t < text.len:
        if p < pattern.len and (pattern[p] == '_' or pattern[p] == text[t]):
            t++, p++
        else if p < pattern.len and pattern[p] == '%':
            star_p = p, star_t = t, p++
        else if star_p >= 0:
            star_t++, t = star_t, p = star_p + 1
        else:
            return false
    while p < pattern.len and pattern[p] == '%': p++
    return p == pattern.len

Related Files

• src/parser.zig — operator enum, WHERE parsing
• src/engine.zig — WHERE clause evaluation
• src/mmap_engine.zig — WHERE evaluation (mmap path)
• src/parallel_mmap.zig — WHERE evaluation (parallel path)