SQL SELECT & Filtering Essentials

SELECT Statement

• Retrieves data from table(s)
• Syntax: SELECT column_A, column_B … FROM table_name;
• Use SELECT * to return all columns

DISTINCT

• Eliminates duplicate rows in result set
• Place DISTINCT immediately after SELECT
• Works on one or multiple columns

WHERE Clause

• Filters rows that satisfy a condition
• Syntax: SELECT … FROM table_name WHERE condition;
• Also valid with UPDATE, DELETE, etc.

ORDER BY

• Sorts results
• Append to query: ORDER BY column_A [, column_B …] ASC | DESC;
• Default order is ascending

LIKE & Wildcards

• Pattern matching inside WHERE
• Wildcards: % (zero + chars), _ (exactly one char)
• Example: WHERE column LIKE 'A%';

Logical Operators (AND / OR / NOT)

• Combine multiple conditions in WHERE
• AND → all conditions must be true
• OR → at least one condition true
• NOT → negates a condition

LIMIT / FETCH

• Restricts number of rows returned → improves performance on large tables
• MySQL: SELECT … LIMIT $n$;
• Oracle: FETCH FIRST $n$ ROWS ONLY or use ROWNUM  n

Aggregate Functions

• Operate on sets, return single value (nulls ignored except by COUNT)
  • MIN() → smallest value
  • MAX() → largest value
  • COUNT() → number of rows
  • SUM() → total of numeric column
  • AVG() → average of numeric column
• Commonly paired with GROUP BY

IN Operator

• Shorthand for multiple OR comparisons
• Syntax: WHERE column IN (value_A, value_B, …);

BETWEEN Operator

• Selects values inside an inclusive range
• Works with numbers, text, dates
• Syntax: WHERE column BETWEEN lower_val AND upper_val;

Aliases (AS)

• Temporary names for columns or tables → improves readability
• Syntax: SELECT column AS alias_name FROM table_name;
• Exist only for the duration of the query