SELECT語句用於從一個或多個表中查詢資料，並根據需要進行過濾、分組、排序等操作。
[ WITH with_query [, ...] ]
SELECT
[ ALL | DISTINCT ] select_expr [, ...]
[ FROM table_reference [, ...] ]
[ WHERE condition ]
[ GROUP BY [ ALL | DISTINCT ] grouping_element [, ...] ]
[ HAVING condition]
[ WINDOW window_name AS (window_spec) [, window_name AS (window_spec)] ...]
[ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] select ]
[ ORDER BY {column_name | expr | position} [ASC | DESC], ... [WITH ROLLUP]]
[LIMIT {[offset,] row_count | row_count OFFSET offset}] table_reference：查詢的資料來源，可以是表、視圖、關聯表或者子查詢。表名和列名不區分大小寫。
表名和列名中如果含有關鍵字或者空格等字元，可以使用反引號（``）將其引起來。
WHERE
WHERE關鍵字後跟BOOLEAN運算式，用於從表中查詢滿足條件的資料。例如，在CUSTOMER表中查詢customer_id為2368的顧客資訊。
SELECT * FROM CUSTOMER where customer_id=2368; ALL和DISTINCT
ALL和DISTINCT關鍵字用於指定查詢結果是否返回重複的行，預設值為ALL，即返回所有匹配的行，DISTINCT將從結果集中重複資料刪除的行。
SELECT id FROM orders;
SELECT DISTINCT id FROM orders; 以下為SELECT中的其他關鍵字用法。