SQL · Theory

sum()

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Theory

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sum()

SUM(marks) adds numbers. NULLs ignored. SELECT SUM(marks) FROM students WHERE city='Pune'; SUM with GROUP BY city.

SUM of no rows is NULL. COALESCE(SUM(marks),0) for 0.

Trap — SUM on a string column. Trap: SUM(DISTINCT …) unless you really mean unique values only.

On the example next to this theory — sum() — aggregate functions collapse many rows into one summary row.

Exam tip

SELECT city, SUM(marks) FROM students GROUP BY city;

sum() — sample query

CREATE TABLE scores (id INTEGER PRIMARY KEY, marks INTEGER);
INSERT INTO scores VALUES (1, 80), (2, 90), (3, 70);
SELECT COUNT(*) AS n, AVG(marks) AS avg_marks, MAX(marks) AS top
FROM scores;

sum() — aggregate functions collapse many rows into one summary row.

Short notes

DefSUM adds numeric values, skips NULL.
RuleCOALESCE(SUM(x),0) if empty should be 0.
TrapSUM of strings.

SQL · Theory

sum()

← All stacks

Theory

119/439

sum()

SUM(marks) adds numbers. NULLs ignored. SELECT SUM(marks) FROM students WHERE city='Pune'; SUM with GROUP BY city.

SUM of no rows is NULL. COALESCE(SUM(marks),0) for 0.

Trap — SUM on a string column. Trap: SUM(DISTINCT …) unless you really mean unique values only.

On the example next to this theory — sum() — aggregate functions collapse many rows into one summary row.

Exam tip

SELECT city, SUM(marks) FROM students GROUP BY city;

sum() — sample query

CREATE TABLE scores (id INTEGER PRIMARY KEY, marks INTEGER);
INSERT INTO scores VALUES (1, 80), (2, 90), (3, 70);
SELECT COUNT(*) AS n, AVG(marks) AS avg_marks, MAX(marks) AS top
FROM scores;

sum() — aggregate functions collapse many rows into one summary row.

Short notes

DefSUM adds numeric values, skips NULL.
RuleCOALESCE(SUM(x),0) if empty should be 0.
TrapSUM of strings.

sum()

sum() — sample query

Short notes

Questions

sum()

sum() — sample query

Short notes

Questions