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KSQL

Expressions

KSQL supports expressions in SELECT, WHERE, ORDER BY, GROUP BY, and HAVING. Anywhere you can put a column name, you can put an expression.

Arithmetic expressions

Standard math works in any clause.

SELECT price * quantity AS total FROM Products
SELECT Revenue - Costs AS profit FROM Deals
SELECT (price * quantity) * (1 - discount / 100) AS net FROM LineItems

Use parentheses to control evaluation order.

String concatenation

Use || or CONCAT() to join strings.

SELECT first_name || ' ' || last_name AS full_name FROM Employees
SELECT Name || ' (' || Status || ')' AS display FROM Customers
SELECT CONCAT(City, ', ', State) AS location FROM Offices

|| and CONCAT() are equivalent for two strings; CONCAT() accepts an arbitrary number of arguments.

CASE WHEN

Conditional logic within a query.

SELECT Name,
    CASE
        WHEN Revenue > 100000 THEN 'Enterprise'
        WHEN Revenue > 50000  THEN 'Mid-Market'
        WHEN Revenue > 10000  THEN 'SMB'
        ELSE 'Starter'
    END AS tier
FROM Customers

Multiple WHEN branches are supported. ELSE is optional (defaults to NULL when omitted).

CASE in WHERE

WHERE CASE
    WHEN Type = 'premium' THEN Priority > 3
    ELSE Priority > 7
END

CASE in ORDER BY

Useful for custom sort orders:

ORDER BY CASE
    WHEN Status = 'urgent' THEN 1
    WHEN Status = 'high'   THEN 2
    WHEN Status = 'medium' THEN 3
    ELSE 4
END ASC

Combining expressions

Expressions nest freely:

-- Profit margin percentage, rounded
SELECT Name,
    ROUND((Revenue - Costs) * 100.0 / Revenue, 1) AS margin_pct
FROM Deals
WHERE Revenue > 0

-- First initial + last name
SELECT UPPER(LEFT(FirstName, 1)) || '. ' || LastName AS short_name
FROM Employees

-- Conditional with date math
SELECT Name,
    IF(DueDate < NOW(), 'OVERDUE', 'On track') AS status,
    DATEDIFF(DueDate, NOW()) AS days_remaining
FROM Tasks
WHERE Status != 'done'

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