SQL · Theory

SQL Interview

Theory

115/439

SQL Interview

A view is a saved SELECT with a name. CREATE VIEW fail_list AS SELECT name, marks FROM students WHERE marks < 40; Then SELECT * FROM fail_list; looks like a table but it runs that query each time (simple views).

Teachers like views for ‘show only what this role should see’. You don’t copy data. If students.marks changes, fail_list changes.

Some views are not updatable (JOIN, GROUP BY). INSERT into fail_list may fail. Don’t promise ‘views always write back’.

Trap — thinking a view stores a snapshot forever. Unless you made a table, it is a query with a nickname.

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

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

Exam tip

CREATE VIEW fail_list AS SELECT … WHERE marks < 40;

SQL Interview — sample query

-- SQL Interview
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;

SQL Interview: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Short notes

DefView = named SELECT. Looks like a table.
RuleData stays in base tables. View refreshes with them.
TrapAssuming every view can INSERT/UPDATE.

SQL · Theory

SQL Interview

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Theory

115/439

SQL Interview

Teachers like views for ‘show only what this role should see’. You don’t copy data. If students.marks changes, fail_list changes.

Some views are not updatable (JOIN, GROUP BY). INSERT into fail_list may fail. Don’t promise ‘views always write back’.

Trap — thinking a view stores a snapshot forever. Unless you made a table, it is a query with a nickname.

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

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

Exam tip

CREATE VIEW fail_list AS SELECT … WHERE marks < 40;

SQL Interview — sample query

-- SQL Interview
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;

SQL Interview: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Short notes

DefView = named SELECT. Looks like a table.
RuleData stays in base tables. View refreshes with them.
TrapAssuming every view can INSERT/UPDATE.

SQL Interview

SQL Interview — sample query

Short notes

Questions

SQL Interview

SQL Interview — sample query

Short notes

Questions