SQL · Theory

CONCAT_WS()()

Theory

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CONCAT_WS()()

CONCAT_WS()() (str) sits in lab hour. Rohit’s job is a CONCAT_WS str dry-run. Write that first.

CONCAT_WS()() (str) on the board: one CONCAT_WS str step Rohit can write from memory. Then say what a CONCAT_WS str dry-run looks like after.

Skip CONCAT_WS()() (str) and skipping the failure case for CONCAT_WS str shows up in lab hour.

Use CONCAT_WS()() (str) when a CONCAT_WS str dry-run must stay clear. If a simpler SQL step works, use that instead.

Don’t do this with CONCAT_WS()() (str): skipping the failure case for CONCAT_WS str. Interviewers spot it in ten seconds.

After CONCAT_WS()() (str), Rohit should still remember skipping the failure case for CONCAT_WS str.

One breath for CONCAT_WS()() (str), then a CONCAT_WS str dry-run, then skipping the failure case for CONCAT_WS str. Sit down.

Exam tip

Board: a CONCAT_WS str dry-run. Dry-run CONCAT_WS()() (str). Name the trap: skipping the failure case for CONCAT_WS str.

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()() (str) — Rohit uses it for a CONCAT_WS str dry-run in lab hour.
RuleCONCAT_WS()() (str) → one CONCAT_WS str step Rohit can write from memory.
RememberCONCAT_WS()() (str) + an index (lab hour).
UseCONCAT_WS()() (str) in lab hour (a CONCAT_WS str dry-run).
TrapCONCAT_WS()() (str) — skipping the failure case for CONCAT_WS str.
ExCONCAT_WS()() (str) → a CONCAT_WS str dry-run.

-- 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()()

CONCAT_WS()() — sample query

Short notes

Questions

CONCAT_WS()()

CONCAT_WS()() — sample query

Short notes

Questions