ภาษา sql เลือกพิเศษ
รหัสตัวอย่าง
หารค่าภาษา SQL
SELECT DISTINCT column1, column1, ..... FROM TABLE
/*DISTINCT WILL NOT SELECT THE SAME VALUES IN SAME COLUMN*/ภาษา sql แทรกสำหรับแต่ละค่าหารค่า
insert into your_table (col1, col2, col3)
select distinct col1, 'newval', col3
from your_tableภาษา sql แทรกสำหรับแต่ละค่าหารค่า
insert into your_table (col1, col2, col3)
select distinct col1, 'newval', col3
from your_tableภาษา sql พิเศษ
This constraint ensures all values in a column are unique.
Example 1 (MySQL): Adds a unique constraint to the id column when
creating a new users table.
CREATE TABLE IF NOT EXISTS users (
id int NOT NULL,
name varchar(255) NOT NULL,
UNIQUE (id)
);
Example 2 (MySQL): Alters an existing column to add a UNIQUE
constraint.
ALTER TABLE users
ADD UNIQUE (id);ภาษา sql พิเศษ
This constraint ensures all values in a column are unique.
Example 1 (MySQL): Adds a unique constraint to the id column when
creating a new users table.
CREATE TABLE IF NOT EXISTS users (
id int NOT NULL,
name varchar(255) NOT NULL,
UNIQUE (id)
);
Example 2 (MySQL): Alters an existing column to add a UNIQUE
constraint.
ALTER TABLE users
ADD UNIQUE (id);ยังไงเพื่อเลือกพิเศษธาตุในภาษา sql
DISTINCT
- select distinct * from employees; ==>
retrieves any row if it has at
least a single unique column.
- select distinct first_name from employees; ==>
retrieves unique names
from table. (removes duplicates)
- select distinct count(*) from employees;
retrieve number of unique rows
if any row has at least a single unique data.ยูนีคนธาตุในภาษา sql
DISTINCT
- select distinct * from employees; ==> retrieves any row if it has at
least a single unique column.
- select distinct first_name from employees; ==> retrieves unique names
from table. (removes duplicates)
- select distinct count(*) from employees; retrieve number of unique rows
if any row has at least a single unique data.หน้าคล้ายกัน
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empty row
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empty row