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Class 12 Structured Query Language (SQL): cheat sheet

CBSE Computer Science. SQL’s DDL and DML commands, data types and constraints, SELECT with WHERE, BETWEEN, IN, LIKE, DISTINCT and ORDER BY, and UPDATE and DELETE.

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Key ideas

DDL commands define the structure (CREATE, ALTER, DROP); DML commands work on the data (INSERT, SELECT, UPDATE, DELETE).
Types: CHAR(n) fixed length, VARCHAR(n) varying, INT, FLOAT, DATE; constraints: PRIMARY KEY, NOT NULL, UNIQUE, DEFAULT.
WHERE picks the rows; ORDER BY sorts them, ascending unless DESC is given.
BETWEEN a AND b includes both ends; IN (…) matches any value in the list; NOT turns a condition round.
A comparison with NULL is never true, so test for it with IS NULL or IS NOT NULL.
DISTINCT removes repeats; UPDATE and DELETE act on every row if there is no WHERE.

Remember

The order of a query	SELECT columns FROM table WHERE condition ORDER BY column
BETWEEN	Salary BETWEEN 30000 AND 50000 means Salary >= 30000 AND Salary <= 50000
LIKE (% any run, _ one character)	'A%' begins with A; '%a' ends with a; '_a%' has a second letter a
NULL	WHERE Bonus IS NULL, never Bonus = NULL
ALTER TABLE … ADD	a new column, in which the existing rows get NULL
DROP or DELETE	DROP TABLE removes the table itself (DDL); DELETE FROM removes rows and keeps the table (DML)

Watch out

Writing = NULL instead of IS NULL.
Thinking BETWEEN leaves out its two end values.
Leaving out WHERE in UPDATE or DELETE, which then changes every row.
Calling DELETE or UPDATE a DDL command: they change data, not structure.

Worked example

Table: EMPLOYEE EmpNo | Name | Dept | Salary | Bonus E1 | Priya | IT | 73000 | 2000 E2 | Arjun | IT | 31000 | 9500 E3 | Vikram | HR | 84000 | 2500 E4 | Meera | IT | 69000 | NULL E5 | Rohan | Accounts | 67000 | 2000 E6 | Zoya | HR | 60000 | 10000 E7 | Dev | Accounts | 45000 | 10000 UPDATE EMPLOYEE SET Salary = Salary + 5000 WHERE Dept NOT IN ('Accounts', 'IT') AND Salary BETWEEN 31000 AND 73000; (a) How many rows does this statement change? (b) What does SELECT SUM(Salary) FROM EMPLOYEE; return after it? (c) Is it a DDL or DML command? Why?

Working: rows where Dept NOT IN ('Accounts', 'IT') AND Salary BETWEEN 31000 AND 73000

Answer: (a) 1; (b) 434000; (c) DML: it changes the data in the table, not its structure.

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