This MySQL Where lesson is for a beginner. Read the short notes, run each example, then change one name or number and run it again.
WHERE keeps the rows that match a condition. The other rows stay in the table and simply are not returned.
Compare text with the same case the column stores, unless you have chosen a case-insensitive collation.
Example 1: Start here
WHERE city = %s with a bound parameter is safer than putting the city inside the SQL text.
rows = [{"name": "Meera", "city": "Chennai"}, {"name": "Arun", "city": "Madurai"}] # a list; the values stay in this order
print([row for row in rows if row["city"] == "Chennai"]) # keep the items that pass the test
Example 2: Filter the rows
MySQL Where is easier to see as a list of records. The filter keeps every row whose seats value is at least 10.
rows = [{"topic": "MySQL Where", "seats": 34}, {"topic": "SQL", "seats": 12}] # a list; the values stay in this order
print([row["topic"] for row in rows if row["seats"] >= 10]) # keep the items that pass the test
Example 3: Update one field
An update changes one field and leaves the rest of the record alone. On a database you would run an UPDATE or an update_one for the same change.
row = {"topic": "MySQL Where", "seats": 34} # a dictionary; each key has one value
row["seats"] = row["seats"] + 1 # change this one field
print(row)
Example 4: Count the matches
Counting matches is the idea behind COUNT(*). Here the number 34 appears twice.
rows = [34, 4, 34] # a list; the values stay in this order
print(sum(1 for value in rows if value == 34)) # keep the items that pass the test
