SQL · Theory

CASE

Theory

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CASE

CASE WHEN marks >= 60 THEN 'A' WHEN marks >= 40 THEN 'B' ELSE 'F' END as a column. Simple CASE: CASE grade WHEN 'A' THEN 5 ELSE 0 END. END is required. Works in SELECT, ORDER BY, sometimes UPDATE SET.

More readable than nested IF(). Always ELSE so NULL doesn’t sneak in.

Trap — forgetting END. Trap: overlapping WHEN without noticing order (first match wins).

On the example next to this theory: CASE: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Exam tip

CASE WHEN marks >= 40 THEN 'Pass' ELSE 'Fail' END

CASE — sample query

-- CASE
CREATE TABLE demo (
  id INTEGER PRIMARY KEY,
  label TEXT NOT NULL,
  qty INTEGER DEFAULT 0
);
INSERT INTO demo (id, label, qty) VALUES
  (1, 'alpha', 2),
  (2, 'beta', 5);
SELECT label, qty FROM demo WHERE qty >= 2 ORDER BY qty DESC;

CASE: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Short notes

DefCASE WHEN … THEN … ELSE … END.
RuleFirst matching WHEN. Always ELSE. Don’t forget END.
TrapNested IF instead of CASE.

-- CASE CREATE TABLE demo ( id INTEGER PRIMARY KEY, label TEXT NOT NULL, qty INTEGER DEFAULT 0 ); INSERT INTO demo (id, label, qty) VALUES (1, 'alpha', 2), (2, 'beta', 5); SELECT label, qty FROM demo WHERE qty >= 2 ORDER BY qty DESC;

CASE

CASE — sample query

Short notes

Questions

CASE

CASE — sample query

Short notes

Questions