SQL · Theory

GROUP_CONCAT()

Theory

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GROUP_CONCAT()

GROUP_CONCAT(name) glues names in a group into one string. SELECT city, GROUP_CONCAT(name ORDER BY name SEPARATOR ', ') FROM students GROUP BY city;

group_concat_max_len truncates long lists (default 1024). Raise it in session if you must, or don’t use GROUP_CONCAT as a data dump.

Trap — GROUP_CONCAT as a substitute for a proper child table in the app.

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

Exam tip

GROUP_CONCAT(name SEPARATOR ', ') per city.

GROUP_CONCAT() — sample query

-- GROUP_CONCAT()
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;

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

Short notes

DefGROUP_CONCAT joins group values into one string.
RuleORDER BY and SEPARATOR inside. Watch max_len.
TrapUsing it instead of a real relation.

-- GROUP_CONCAT() 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;

GROUP_CONCAT()

GROUP_CONCAT() — sample query

Short notes

Questions

GROUP_CONCAT()

GROUP_CONCAT() — sample query

Short notes

Questions