WHERE

WHERE decides which rows survive. It runs after FROM and before SELECT, which is why you can filter on a column you never return.

SELECT name
FROM customers
WHERE country = 'GE' AND credit > 0;

AND binds tighter than OR, so parenthesise the moment you mix them. WHERE a AND b OR c almost never means what the person typing it meant.

country IN ('GE', 'PL') is shorthand for country = 'GE' OR country = 'PL'. LIKE matches a pattern: % stands for any run of characters, including none, and _ for exactly one, so name LIKE 'Ni%' keeps every name that starts with Ni.

Try it

NULL is not a value

NULL means unknown. Comparing an unknown to anything gives another unknown, and a WHERE clause keeps a row only when the answer is true.

So WHERE credit = NULL returns nothing, ever. Not an error, not a warning: nothing. The same is true of <> NULL.

SELECT name FROM customers WHERE credit IS NULL;

IS NULL and IS NOT NULL are the only comparisons that answer the question. Reach for them the moment a column is nullable.

= NULL reads naturally and fails silently, which is why people write it with confidence, in code and in interviews.

Common mistakes

• Mixing AND and OR without parentheses. "Georgian or Polish customers with more than 400 credit" written as country = 'GE' OR country = 'PL' AND credit > 400 reads as GE OR (PL AND credit > 400), so every Georgian comes back — including one whose credit is unknown.

• Expecting <> to keep the NULL rows. credit <> 500 is unknown for a NULL credit, so that row is dropped too: four rows have a credit that is not 500 as far as a person is concerned, and the count above says three.
• Quoting a string with double quotes. In standard SQL "GE" is a column name, not text; strings take single quotes.
• Filtering on an alias from the SELECT list. WHERE runs before SELECT, so the alias does not exist yet. Repeat the expression.

What interviewers ask

• Why does WHERE x <> 5 miss rows where x is NULL? The comparison is unknown, not true, and WHERE keeps only true. Add OR x IS NULL if those rows belong in the answer.
• Which binds tighter, AND or OR? AND. Parenthesise whenever both appear.
• Can WHERE use a column you do not select? Yes — it runs before SELECT, against every column of the tables in .