15_nested_loops.py

#======================================================================================
#                                   NESTED LOOPS
#--------------------------------------------------------------------------------------
# A nested loop means one loop inside another loop
# if:
#   -outer loop runs m times
#   -inner loop runs n times
# Total operations = m * n
#======================================================================================
for x in range(3): # outer loop
    for y in range(2): # inner loop
        print(x,y)

#--------------------------------------------------------------------------------------
# Real world uses cases
#--------------------------------------------------------------------------------------
# why we need nested loops?
#   1) crossing data
#   2) navigate hierarchy

#--------------------------------------------------------------------------------------
# crossing data
#--------------------------------------------------------------------------------------
colors = ["Red", "Blue", "Green"]
sizes = ["S","M","L","XL"]
for color in colors:
    for size in sizes:
        print(f"{color} - size {size}")

#--------------------------------------------------------------------------------------
# navigate hierarchy
#--------------------------------------------------------------------------------------
years = [2025, 2026]
months = ["Jan", "Feb"]
days = range(1,29)
for year in years:
    for month in months:
        for day in days:
            print(f"report_{year}_{month}_{day}.csv")

# SELECT COUNT(*) FROM customers WHERE  id IS NULL;
tables = ["customers", "orders", "products", "prices"]
columns = ["id", "create_date"]
for table in tables:
    for column in columns:
        print(f"SELECT COUNT(*) FROM {table} WHERE {column} IS NULL;")