MySQL CHAR和VARCHAR數據類型介紹

發布時間：2020-08-07 01:18:50 來源：ITPUB博客閱讀：229 作者：feelpurple 欄目：MySQL數據庫

CHAR類型是固定長度的，長度范圍為0到255。如果存儲的值不覆蓋字段長度，存儲數據的其他的部分會在值的右邊自動添加空格。

VARCHAR字段中的值是可變長度的字符串，長度范圍為0到65535。VARCHAR字段的值以1字節或2字節的長度前綴加上數據的方式存儲。長度前綴指示了字段存儲的字節數，如果一個字段存儲的值不超過255字節，它使用一個長度前綴，如果字段存儲的值超過了255字節，它使用兩個長度前綴。

如果沒有啟用嚴格SQL模式，分配給CHAR或VARCHAR字段超過字段最大長度的值，這個值會被截斷保存到數據庫中，并會產生一個警告。對于VARCHAR字段來說，不管使用什么樣的SQL模式，分配給VARCHAR字段超過字段最大長度的值，在值插入前，超過字段最大長度的部分會被截斷，并會產生一個警告。對于CHAR字段來說，不管使用什么樣的SQL模式，超過字段最大長度的部分會被截斷，并隱式插入到數據庫中。

--測試①

MariaDB [test]> show variables like '%sql%mode%';
+---------------+--------------------------------------------+
| Variable_name | Value |
+---------------+--------------------------------------------+
| sql_mode | NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
+---------------+--------------------------------------------+
1 row in set (0.26 sec)

MariaDB [test]> drop table vc;
Query OK, 0 rows affected (0.04 sec)

MariaDB [test]> CREATE TABLE vc (v VARCHAR(4), c CHAR(4));
Query OK, 0 rows affected (0.54 sec)

MariaDB [test]> INSERT INTO vc VALUES ('ab ', 'ab ');
Query OK, 1 row affected (0.14 sec)

--查看插入的數據中是否保留空格

MariaDB [test]> SELECT CONCAT('(', v, ')'), CONCAT('(', c, ')') FROM vc;
+---------------------+---------------------+
| CONCAT('(', v, ')') | CONCAT('(', c, ')') |
+---------------------+---------------------+
| (ab ) | (ab) |
+---------------------+---------------------+
1 row in set (0.00 sec)

MariaDB [test]> select length(v),length(c) from vc;
+-----------+-----------+
| length(v) | length(c) |
+-----------+-----------+
| 4 | 2 |
+-----------+-----------+
1 row in set (0.00 sec)

--插入超過字段最大值的數據到表中

MariaDB [test]> insert into vc values('abcdefg','abcedeg');
Query OK, 1 row affected, 2 warnings (0.03 sec)

MariaDB [test]> show warnings;
+---------+------+----------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------+
| Warning | 1265 | Data truncated for column 'v' at row 1 |
| Warning | 1265 | Data truncated for column 'c' at row 1 |
+---------+------+----------------------------------------+
2 rows in set (0.00 sec)

MariaDB [test]> select * from vc;
+------+------+
| v | c |
+------+------+
| ab | ab |
| abcd | abce |
+------+------+
2 rows in set (0.00 sec)

--測試②

MariaDB [test]> CREATE TABLE names (myname CHAR(10));
Query OK, 0 rows affected (0.13 sec)

MariaDB [test]> INSERT INTO names VALUES ('Monty');
Query OK, 1 row affected (0.00 sec)

--查看表中數據在WHERE條件中使用=的條件中帶空格的執行情況

MariaDB [test]> SELECT myname = 'Monty', myname = 'Monty ' FROM names;
+------------------+--------------------+
| myname = 'Monty' | myname = 'Monty ' |
+------------------+--------------------+
| 1 | 1 |
+------------------+--------------------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names;
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> select length(myname) from names;
+----------------+
| length(myname) |
+----------------+
| 5 |
+----------------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname = 'Monty';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname = 'Monty ';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname = 'Monty ';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname = 'Monty ';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname = 'Monty ';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname = 'Monty ';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname = 'Monty ';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname = 'Monty ';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

--查看表中的數據在like條件下的執行情況

MariaDB [test]> SELECT myname LIKE 'Monty', myname LIKE 'Monty ' FROM names;
+---------------------+-----------------------+
| myname LIKE 'Monty' | myname LIKE 'Monty ' |
+---------------------+-----------------------+
| 1 | 0 |
+---------------------+-----------------------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname like '%Monty%';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname like 'Monty%';
+--------+
| myname |
+--------+
| Monty |
+--------+
1 row in set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname like 'Monty %';
Empty set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname like 'Monty %';
Empty set (0.00 sec)

MariaDB [test]> SELECT * FROM names where myname like 'Monty %';
Empty set (0.00 sec)

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