SQL Query

Jun 28, 2023

• Related: Relational Algebra

SELECT [DISTINCT] <column expression list>  
FROM <single table>  
[WHERE <predicate>]  
[GROUP BY <column list>  
[HAVING <predicate>] ]  
[ORDER BY <column list>]  
[LIMIT <integer>];

• SQL SELECT
• SQL WHERE
• SQL ORDER BY
• SQL GROUP BY

• AVG()
• COUNT()
• SUM()

• .schema
• .tables
• SQL Null
  • represents an “unknown” or “missing” value.
  • If you do any calculation with NULL, you’ll just get NULL. For instance if x is NULL, then x > 3, 1 = x, and x + 4 all evaluate to NULL. Even x = NULL would evaluate to NULL
  • if you want to check whether x is NULL, then write x IS NULL or x IS NOT NULL instead.
  • when the comparison involves NULL values. UNKNOWN
  • NULL is falsey, meaning that WHERE NULL is just like WHERE FALSE. The row in question does not get included.
  • NULL column values are ignored by aggregate functions.
  • SQL Set
    • UNION
    • INTERSECT
    • EXCEPT
    • <ALL>: All possible value. (Allow Duplicates, Optional)
• SQL View
• SQL Common Table Expression
• SQL Testing
• Query Optimization