Bagaimana cara mengubah nilai default di mysql?

Question

Ringkasan. dalam tutorial ini, Anda akan belajar tentang kendala DEFAULT MySQL dan cara menggunakannya secara efektif

Table of Contents Show

Pengantar batasan DEFAULT MySQL
Contoh batasan MySQL DEFAULT
Menambahkan kendala DEFAULT ke kolom
Menghapus kendala DEFAULT dari kolom
Bagaimana cara mengubah nilai string DEFAULT di SQL?
Bisakah kita mengubah nilai default di SQL?
Bagaimana cara mengatur nilai default 0 di MySQL?
Bagaimana cara mengatur nilai default false di MySQL?

Pengantar batasan DEFAULT MySQL

Batasan MySQL

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

_1 memungkinkan Anda menentukan nilai default untuk kolom. Inilah sintaks dari kendala

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

1

column_name data_type DEFAULT default_value;
Code language: SQL (Structured Query Language) (sql)

Dalam sintaks ini, Anda menentukan kata kunci

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

1 diikuti dengan nilai default untuk kolom. Tipe nilai default cocok dengan tipe data kolom

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

4 harus berupa konstanta literal, mis. g. , angka atau string. Itu tidak bisa menjadi fungsi atau ekspresi. Namun, MySQL memungkinkan Anda untuk mengatur tanggal dan waktu saat ini (

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

5) ke kolom

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

6 dan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

7

Saat Anda menentukan kolom tanpa batasan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

_8, kolom tersebut secara implisit akan menggunakan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

9 sebagai nilai default

Jika kolom memiliki batasan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

1 dan pernyataan

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)



Code language: plaintext (plaintext)

1 atau

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

2 tidak memberikan nilai untuk kolom tersebut, MySQL akan menggunakan nilai default yang ditentukan dalam batasan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

1

Biasanya, Anda menyetel batasan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

_1 untuk kolom saat Anda membuat tabel. MySQL juga memungkinkan Anda untuk menambahkan batasan default ke kolom tabel yang ada. Jika Anda tidak ingin menggunakan nilai default untuk kolom, Anda dapat menghapus batasan default

Contoh batasan MySQL DEFAULT

Contoh berikut membuat tabel baru bernama

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

5 dengan empat kolom

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

6,

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

7,

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

8, dan

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

9

CREATE TABLE cart_items 
(
    item_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    quantity INT NOT NULL,
    price DEC(5,2) NOT NULL,
    sales_tax DEC(5,2) NOT NULL DEFAULT 0.1,
    CHECK(quantity > 0),
    CHECK(sales_tax >= 0) 
);
Code language: SQL (Structured Query Language) (sql)

Kolom

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

_9 memiliki nilai default 0. 1 (10%). Pernyataan berikut menunjukkan tabel

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

_5

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

Keluaran

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

Pernyataan

INSERT INTO cart_items(name, quantity, price)
VALUES('Keyboard', 1, 50);
Code language: SQL (Structured Query Language) (sql)

_2 berikut menambahkan item baru ke tabel

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

5

INSERT INTO cart_items(name, quantity, price)
VALUES('Keyboard', 1, 50);
Code language: SQL (Structured Query Language) (sql)

Dalam contoh ini, pernyataan

INSERT INTO cart_items(name, quantity, price)
VALUES('Keyboard', 1, 50);
Code language: SQL (Structured Query Language) (sql)

_2 tidak memberikan nilai untuk kolom

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

9. Kolom

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

_9 menggunakan nilai default yang ditentukan dalam batasan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

1

SELECT * FROM cart_items;
Code language: SQL (Structured Query Language) (sql)

Keluaran

+---------+----------+----------+-------+-----------+
| item_id | name     | quantity | price | sales_tax |
+---------+----------+----------+-------+-----------+
|       1 | Keyboard |        1 | 50.00 |      0.10 |
+---------+----------+----------+-------+-----------+
1 row in set (0.00 sec)
Code language: plaintext (plaintext)

Juga, Anda dapat secara eksplisit menggunakan kata kunci

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

1 saat Anda memasukkan baris baru ke dalam tabel

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

5

INSERT INTO cart_items(name, quantity, price, sales_tax)
VALUES('Battery',4, 0.25 , DEFAULT);



Code language: SQL (Structured Query Language) (sql)

Dalam hal ini, kolom

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

_9 mengambil nilai default

SELECT * FROM cart_items;
Code language: SQL (Structured Query Language) (sql)

Keluaran

+---------+----------+----------+-------+-----------+
| item_id | name     | quantity | price | sales_tax |
+---------+----------+----------+-------+-----------+
|       1 | Keyboard |        1 | 50.00 |      0.10 |
|       2 | Battery  |        4 |  0.25 |      0.10 |
+---------+----------+----------+-------+-----------+
2 rows in set (0.01 sec)
Code language: plaintext (plaintext)

Menambahkan kendala DEFAULT ke kolom

Untuk menambahkan batasan default ke kolom tabel yang sudah ada, Anda menggunakan pernyataan

SELECT * FROM cart_items;
Code language: SQL (Structured Query Language) (sql)

1

CREATE TABLE cart_items 
(
    item_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    quantity INT NOT NULL,
    price DEC(5,2) NOT NULL,
    sales_tax DEC(5,2) NOT NULL DEFAULT 0.1,
    CHECK(quantity > 0),
    CHECK(sales_tax >= 0) 
);
Code language: SQL (Structured Query Language) (sql)

0

Contoh berikut menambahkan batasan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

1 ke kolom

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

8 dari tabel

SELECT * FROM cart_items;
Code language: SQL (Structured Query Language) (sql)

4

CREATE TABLE cart_items 
(
    item_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    quantity INT NOT NULL,
    price DEC(5,2) NOT NULL,
    sales_tax DEC(5,2) NOT NULL DEFAULT 0.1,
    CHECK(quantity > 0),
    CHECK(sales_tax >= 0) 
);
Code language: SQL (Structured Query Language) (sql)

1

Jika Anda mendeskripsikan tabel

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

_5, Anda akan melihat perubahannya

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

Keluaran

CREATE TABLE cart_items 
(
    item_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    quantity INT NOT NULL,
    price DEC(5,2) NOT NULL,
    sales_tax DEC(5,2) NOT NULL DEFAULT 0.1,
    CHECK(quantity > 0),
    CHECK(sales_tax >= 0) 
);
Code language: SQL (Structured Query Language) (sql)

3

Pernyataan berikut menyisipkan baris baru ke dalam tabel

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

5 tanpa menentukan nilai untuk kolom

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

8

CREATE TABLE cart_items 
(
    item_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    quantity INT NOT NULL,
    price DEC(5,2) NOT NULL,
    sales_tax DEC(5,2) NOT NULL DEFAULT 0.1,
    CHECK(quantity > 0),
    CHECK(sales_tax >= 0) 
);
Code language: SQL (Structured Query Language) (sql)

4

Nilai kolom

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

_8 akan default ke 1

SELECT * FROM cart_items;
Code language: SQL (Structured Query Language) (sql)

Keluaran

CREATE TABLE cart_items 
(
    item_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    quantity INT NOT NULL,
    price DEC(5,2) NOT NULL,
    sales_tax DEC(5,2) NOT NULL DEFAULT 0.1,
    CHECK(quantity > 0),
    CHECK(sales_tax >= 0) 
);
Code language: SQL (Structured Query Language) (sql)

6

Menghapus kendala DEFAULT dari kolom

Untuk menghapus batasan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

1 dari kolom, Anda menggunakan pernyataan

+---------+----------+----------+-------+-----------+
| item_id | name     | quantity | price | sales_tax |
+---------+----------+----------+-------+-----------+
|       1 | Keyboard |        1 | 50.00 |      0.10 |
+---------+----------+----------+-------+-----------+
1 row in set (0.00 sec)
Code language: plaintext (plaintext)

0

CREATE TABLE cart_items 
(
    item_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    quantity INT NOT NULL,
    price DEC(5,2) NOT NULL,
    sales_tax DEC(5,2) NOT NULL DEFAULT 0.1,
    CHECK(quantity > 0),
    CHECK(sales_tax >= 0) 
);
Code language: SQL (Structured Query Language) (sql)

7

Contoh berikut menghilangkan batasan

DESC cart_items;
Code language: SQL (Structured Query Language) (sql)

1 dari kolom

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

8 pada tabel

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)
Code language: plaintext (plaintext)

5

Bagaimana cara mengubah nilai string DEFAULT di SQL?

Pilih kolom yang ingin Anda tentukan nilai defaultnya. Di tab Properti Kolom, masukkan nilai default baru di properti Default Value atau Binding . Untuk memasukkan nilai default numerik, masukkan angka. Untuk objek atau fungsi masukkan namanya.

Bisakah kita mengubah nilai default di SQL?

Anda dapat menggunakan pernyataan ALTER TABLE untuk menambah, mengubah, atau menghapus nilai default untuk kolom .

Bagaimana cara mengatur nilai default 0 di MySQL?

Untuk tipe numerik, defaultnya adalah 0 , dengan pengecualian untuk tipe integer atau floating-point yang dideklarasikan dengan atribut AUTO_INCREMENT, defaultnya adalah nilai berikutnya dalam urutan. Untuk tipe tanggal dan waktu selain TIMESTAMP , defaultnya adalah nilai "nol" yang sesuai untuk tipe tersebut

Bagaimana cara mengatur nilai default false di MySQL?

Untuk membuat kolom dengan 'false' sebagai nilai default, kita dapat menggunakan konsep “default” pada saat pembuatan tabel. Note − 0 represents false and 1 represents true. Creating a table using “default” false.

kode mysql