SQL · Theory

CONCAT_WS()()

Theory

132/439

CONCAT_WS()()

CONCAT_WS() glues strings with a separator and skips NULLs. You use it in SELECT (and sometimes WHERE/UPDATE). It does not change stored data unless you UPDATE … SET col = CONCAT_WS(col).

Board: SELECT CONCAT_WS(' ', first, last) FROM students; Run it. Say the result out loud. If the column is NULL, the result is often NULL — wrap with IFNULL if you need a blank instead.

String indexes in MySQL start at 1, not 0 (SUBSTR, INSTR, LEFT). First argument is the separator, not a name.

Interview — name CONCAT_WS, one argument list, one example on the students table. Don’t recite every string function.

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

Exam tip

Say what CONCAT_WS returns, then SELECT CONCAT_WS(' ', first, last) FROM students;

CONCAT_WS()() — sample query

-- CONCAT_WS()()
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;

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

Short notes

DefCONCAT_WS() glues strings with a separator and skips NULLs.
RuleSELECT CONCAT_WS(' ', first, last) FROM students;
TrapFirst argument is the separator, not a name.

SQL · Theory

CONCAT_WS()()

← All stacks

Theory

132/439

CONCAT_WS()()

CONCAT_WS() glues strings with a separator and skips NULLs. You use it in SELECT (and sometimes WHERE/UPDATE). It does not change stored data unless you UPDATE … SET col = CONCAT_WS(col).

Board: SELECT CONCAT_WS(' ', first, last) FROM students; Run it. Say the result out loud. If the column is NULL, the result is often NULL — wrap with IFNULL if you need a blank instead.

String indexes in MySQL start at 1, not 0 (SUBSTR, INSTR, LEFT). First argument is the separator, not a name.

Interview — name CONCAT_WS, one argument list, one example on the students table. Don’t recite every string function.

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

Exam tip

Say what CONCAT_WS returns, then SELECT CONCAT_WS(' ', first, last) FROM students;

CONCAT_WS()() — sample query

-- CONCAT_WS()()
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;

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

Short notes

DefCONCAT_WS() glues strings with a separator and skips NULLs.
RuleSELECT CONCAT_WS(' ', first, last) FROM students;
TrapFirst argument is the separator, not a name.

CONCAT_WS()()

CONCAT_WS()() — sample query

Short notes

Questions

CONCAT_WS()()

CONCAT_WS()() — sample query

Short notes

Questions