🗄️ SQL Commands Cheat Sheet¶

📋 Quick Reference¶

DDL (Data Definition Language)¶

CREATE TABLE¶

CREATE TABLE table_name (
    column1 datatype constraints,
    column2 datatype constraints,
    ...
);

ALTER TABLE¶

ALTER TABLE table_name
ADD column_name datatype;

DROP TABLE¶

DROP TABLE table_name;

DML (Data Manipulation Language)¶

INSERT¶

INSERT INTO table_name (column1, column2, ...)
VALUES (value1, value2, ...);

UPDATE¶

UPDATE table_name
SET column1 = value1, column2 = value2
WHERE condition;

DELETE¶

DELETE FROM table_name
WHERE condition;

SELECT Queries¶

Basic SELECT¶

SELECT column1, column2
FROM table_name;

SELECT with WHERE¶

SELECT *
FROM table_name
WHERE condition;

SELECT with ORDER BY¶

SELECT *
FROM table_name
ORDER BY column_name [ASC|DESC];

🔍 Filtering & Conditions¶

Comparison Operators¶

= : Equal to
!= or <> : Not equal to
> : Greater than
< : Less than
>= : Greater than or equal to
<= : Less than or equal to

Logical Operators¶

AND : All conditions must be true
OR : At least one condition must be true
NOT : Negates the condition

Pattern Matching¶

-- Starts with
WHERE column_name LIKE 'prefix%'

-- Ends with
WHERE column_name LIKE '%suffix'

-- Contains
WHERE column_name LIKE '%substring%'

-- Single character wildcard
WHERE column_name LIKE 'prefix_'

🤝 JOINs¶

INNER JOIN¶

SELECT columns
FROM table1
INNER JOIN table2 ON table1.common_column = table2.common_column;

LEFT JOIN¶

SELECT columns
FROM table1
LEFT JOIN table2 ON table1.common_column = table2.common_column;

RIGHT JOIN¶

SELECT columns
FROM table1
RIGHT JOIN table2 ON table1.common_column = table2.common_column;

📊 Aggregate Functions¶

Common Aggregates¶

SELECT COUNT(column_name) FROM table_name;
SELECT SUM(column_name) FROM table_name;
SELECT AVG(column_name) FROM table_name;
SELECT MIN(column_name) FROM table_name;
SELECT MAX(column_name) FROM table_name;

GROUP BY¶

SELECT column_name, COUNT(*)
FROM table_name
GROUP BY column_name;

HAVING¶

SELECT column_name, COUNT(*)
FROM table_name
GROUP BY column_name
HAVING COUNT(*) > 1;

🎯 Common Patterns¶

Pagination¶

SELECT *
FROM table_name
LIMIT 10 OFFSET 20;

Subquery¶

SELECT *
FROM table_name
WHERE column_name IN (SELECT column_name FROM another_table WHERE condition);

CASE Statement¶

SELECT column_name,
    CASE
        WHEN condition1 THEN result1
        WHEN condition2 THEN result2
        ELSE default_result
    END AS new_column
FROM table_name;

🔍 Debugging Checklist¶

Table and column names are correct
Quotes are used properly for strings
WHERE clause is included for UPDATE/DELETE
JOIN conditions are correct
Aggregate functions have GROUP BY when needed
Subqueries return single values when required

⚡ Performance Tips¶

Optimization Guidelines¶

✅ Use indexes on frequently queried columns
✅ SELECT only needed columns, avoid SELECT *
✅ Use appropriate JOIN types
✅ Use LIMIT for large result sets
✅ Consider EXISTS instead of IN for subqueries

Common Pitfalls¶

❌ Missing WHERE clause in UPDATE/DELETE
❌ Cartesian products in JOINs
❌ NULL comparisons with = (use IS NULL)
❌ String comparisons without proper casing

Last updated: Phase 3 Implementation