SQL · Theory

IS NOT NULL

Theory

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IS NOT NULL

WHERE marks IS NOT NULL; rows that actually have a marks value (including 0). Use it when you want ‘exam written’ vs ‘absent’.

COUNT(marks) is similar to counting IS NOT NULL on that column.

Trap — IS NOT NULL and then still using the column in a way that assumes it cannot be 0.

On the example next to this theory: IS NOT NULL: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Exam tip

WHERE marks IS NOT NULL;

IS NOT NULL — sample query

-- IS NOT NULL
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;

IS NOT NULL: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Short notes

DefIS NOT NULL = column has a value.
Rule0 is a value. NULL is not.
TrapTreating 0 as missing.

-- IS NOT NULL 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;

IS NOT NULL

IS NOT NULL — sample query

Short notes

Questions

IS NOT NULL

IS NOT NULL — sample query

Short notes

Questions