Explain the full syntax of the SELECT statement in a relational database.

The SQL SELECT operator is used to select data from one or more tables. Its full syntax includes many optional parts:

SELECT [ALL | DISTINCT]
    select_expr, select_expr, ...
FROM
    table_references
[WHERE
    where_condition]
[GROUP BY
    {col_name | expr | position}
    [ASC | DESC], ... [WITH ROLLUP]]
[HAVING
    having_condition]
[ORDER BY
    {col_name | expr | position}
    [ASC | DESC], ...]
[LIMIT
    [offset,] row_count | row_count OFFSET offset]
    [FOR UPDATE | LOCK IN SHARE MODE];

Let's go over the main parts:

• SELECT [ALL | DISTINCT] select_expr, ...:

  • SELECT: Keyword indicating data selection.
  • ALL: (Default) Includes all found rows, including duplicates.
  • DISTINCT: Removes duplicate rows from the result.
  • select_expr: Expression defining selected columns or computed values (e.g., column_name, function(column), column1 + column2, * for all columns). Aliases can be used with AS.

• FROM table_references:

  • FROM: Keyword indicating the data source.
  • table_references: List of tables from which data is selected. Can include single tables, views, or join results (JOIN). Types of joins include: INNER JOIN, LEFT [OUTER] JOIN, RIGHT [OUTER] JOIN, FULL [OUTER] JOIN, CROSS JOIN.

• [WHERE where_condition]:

  • WHERE: Keyword for filtering rows based on a condition.
  • where_condition: Logical expression using comparison operators (=, !=, >, <, >=, <=), logical operators (AND, OR, NOT), range operators (BETWEEN), list operators (IN), pattern matching (LIKE), and NULL checks (IS NULL, IS NOT NULL).

• [GROUP BY {col_name | expr | position} [ASC | DESC], ... [WITH ROLLUP]]:

  • GROUP BY: Groups rows with identical values in specified columns for aggregate functions (COUNT, SUM, AVG, MIN, MAX).
  • col_name | expr | position: Column, expression, or column position for grouping.
  • ASC | DESC: Optional, defines sorting order within the group (usually used with ORDER BY).
  • WITH ROLLUP: Optional, adds rows with total values (for MySQL).

• [HAVING having_condition]:

  • HAVING: Filters groups created by GROUP BY based on a condition. Unlike WHERE, HAVING can use aggregate functions in the condition.

• [ORDER BY {col_name | expr | position} [ASC | DESC], ...]:

  • ORDER BY: Sorts the resulting set of rows.
  • col_name | expr | position: Column, expression, or column position for sorting.
  • ASC: (Default) Ascending order.
  • DESC: Descending order.

• [LIMIT [offset,] row_count | row_count OFFSET offset]:

  • LIMIT: Limits the number of returned rows. Can be used for pagination.
  • offset: Offset from the start of the result set (number of rows to skip).
  • row_count: Maximum number of rows to return. Syntax may vary slightly across DBMSs (e.g., TOP in SQL Server).

• [FOR UPDATE | LOCK IN SHARE MODE]:

  • Optional row locks used in transactions to control concurrent access.
  • FOR UPDATE: Obtains an exclusive lock on selected rows, preventing other transactions from modifying them.
  • LOCK IN SHARE MODE: Obtains a shared lock, allowing other transactions to read but not modify the rows (for MySQL). Similar mechanisms may be used in other DBMSs (e.g., FOR SHARE).

The order of applying parts of the SELECT statement during a DBMS query execution is usually:

1. FROM
2. JOIN
3. WHERE
4. GROUP BY
5. HAVING
6. SELECT
7. DISTINCT
8. ORDER BY
9. LIMIT
10. FOR UPDATE / LOCK IN SHARE MODE