Retrieving Individual Columns . 检索单个列

  1. SELECT Name 
  2. FROM country

Retrieving Multiple Columns 检索多个列

SELECT Name, Continent, Population
FROM country

Retrieving All Columns 检索所有列

SELECT *
FROM countrylanguage

Retrieving Distinct Columns 检索不同的列

SELECT DISTINCT CountryCode
FROM countrylanguage

Limiting Results 极限结果

MySQL，PostgreSQL，SQLite

/* first five rows */
SELECT Name
FROM country
LIMIT 5

/* from the 6th to the 10th rows */
SELECT Name
FROM country
LIMIT 5, 5

Ms SQL，Access

SELECT TOP 5 Name
FROM country

DB2

SELECT Name
FROM country
FETCH FIRST 5 ROWS ONLY

Oracle

SELECT Name
FROM country
WHERE ROWNUM <= 5

Interbase，Firebird

/* first five rows */
SELECT Name
FROM country
ROWS 5

/* from the 6th to the 10th rows */
SELECT Name
FROM country
ROWS 6 TO 10

Using Comments 使用注释

# first comment
SELECT Name -- second comment
FROM country
/* third
comment */

Heavy life list 重命名列

SELECT device_id as user_infors_example
FROM user_profile

Select Clause Ordering 选择子句排序

Clause Ordering:

Предложение Описание Необходимость
SELECT Columns or expressions to be returned Yes
FROM Table or subquery to retrieve data from Yes
WHERE Row-level filtring No
GROUP BY Group specification Only if calculating aggregates by group
HAVING Group-level filtring No
ORDER BY Output sort order No