SQL · Theory

avg()

Theory

120/439

avg()

AVG(marks) = SUM/COUNT of non-NULL marks. Absentees (NULL) don’t pull the average down. If they should count as 0, AVG(IFNULL(marks,0)).

ROUND(AVG(marks), 2) for two decimals.

Trap — averaging in PHP after SELECT * of 1 lakh rows.

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

Exam tip

AVG(marks) ignores NULL. AVG(IFNULL(marks,0)) includes as 0.

avg() — 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;

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

Short notes

DefAVG is mean of non-NULL values.
RuleIFNULL if missing should count as 0.
TrapNULL absentees silently excluded.

SQL · Theory

avg()

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Theory

120/439

avg()

AVG(marks) = SUM/COUNT of non-NULL marks. Absentees (NULL) don’t pull the average down. If they should count as 0, AVG(IFNULL(marks,0)).

ROUND(AVG(marks), 2) for two decimals.

Trap — averaging in PHP after SELECT * of 1 lakh rows.

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

Exam tip

AVG(marks) ignores NULL. AVG(IFNULL(marks,0)) includes as 0.

avg() — 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;

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

Short notes

DefAVG is mean of non-NULL values.
RuleIFNULL if missing should count as 0.
TrapNULL absentees silently excluded.

avg()

avg() — sample query

Short notes

Questions

avg()

avg() — sample query

Short notes

Questions