SQL Basics

UNION – Combine Query Results

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Combines the results of two or more SELECT queries into one result set.

SELECT name FROM Patients
UNION
SELECT name FROM Doctors;

Notes:

• Number and type of columns must match.
• Returns only unique values.
• Use UNION ALL if you want to keep duplicates.

EXISTS – Check If Data Exists

Returns TRUE if the subquery returns any result.

SELECT * FROM Patients
WHERE EXISTS (
  SELECT * FROM Appointments
  WHERE Patients.PatientID = Appointments.PatientID
);

Notes:

• It checks if data exists, not what the data is.
• If the subquery returns something, the row is selected.

ANY – Compare with Any Value

Returns TRUE if the condition is true for at least one value in the subquery.

SELECT * FROM Doctors
WHERE Salary > ANY (
  SELECT Salary FROM Doctors WHERE Department = 'Cardiology'
);

Notes:

• If one or more values match the condition, the row is selected.
• Similar to using IN for equality: = ANY (...).

ALL – Compare with All Values

Returns TRUE if the condition is true for all values in the subquery.

SELECT * FROM Doctors
WHERE Salary > ALL (
  SELECT Salary FROM Doctors WHERE Department = 'Cardiology'
);

Notes:

• The condition must be true for every value in the subquery.
• If even one value does not match, the row is not selected.

SELECT INTO – Copy Data to a New Table

Creates a new table and copies data from a query into it.

SELECT * INTO NewDoctors
FROM Doctors
WHERE Department = 'Neurology';

Notes:

• The new table should not already exist.
• To add data to an existing table, use INSERT INTO.

INSERT INTO SELECT

It copies data from one existing table into another existing table. It does not create a new table.

INSERT INTO OldPatients (Name, BirthDate)
SELECT Name, BirthDate
FROM Patients
WHERE YEAR(BirthDate) < 1990;

ISNULL

Checks if a value is NULL.
If it is, replaces it with another value.

SELECT ISNULL(column, replacement_value) AS result
FROM table;

Often used to avoid nulls in calculations or display defaults like 0, '', or 'N/A'.

CASE

Works like an if-else in SQL.
Returns different values based on conditions.

CASE
 WHEN condition1 THEN result1
 WHEN condition2 THEN result2
 WHEN conditionN THEN resultN
 ELSE result
END;

Useful for custom labels, categories, or conditional logic in queries.