Table of contents
1.1. Commonly used date functions
1.4. current_timestamp function
2.1. Commonly used string functions
2.12.ltrim, rtrim and trim functions
3.1. Commonly used mathematical functions
1. Date function
Aggregate functions can only appear in the select statement, and the date function here can appear in both the select statement and the where filter statement.
1.1. Commonly used date functions
Commonly used date functions are as follows:
1.2. current_date function
The current_date function is used to get the current date. as follows:
1.3. current_time function
The current_time function is used to get the current time. as follows:
1.4. current_timestamp function
The current_timestamp function is used to obtain the current timestamp and display it in date and time format. as follows:
1.5. now function
The now function is used to get the current date and time. as follows:
1.6. date function
The date function is used to get the current date and time. as follows:
1.7. date_add function
The date_add function is used to add a date or time based on a date. as follows:
If the date/time added in the date_add function is negative, it is equivalent to subtracting the date/time from the date. as follows:
1.8. date_sub function
The date_sub function is used to subtract a date or time from a date. as follows:
If the date/time subtracted in the date_sub function is negative, it is equivalent to adding the date/time to the base of the date. as follows:
1.9. datediff function
The datediff function is used to get the difference between two dates in days. as follows:
1.10. Comprehensive case
Create a comment table, which contains self-increasing primary key id, nickname, comment content and comment time. as follows:
Insert some data into the table, and specify the comment time directly through the now function when inserting. as follows:
When displaying comment information, if you only want to display the date of the comment but not the time of the comment, you can use the date function to intercept the date part of the sendtime when querying the sendtime field for display. as follows:
Insert some data into the table from time to time. as follows:
When displaying comment information, if you want to query the comment information published within 2 minutes, you actually need to filter out comments whose comment time plus 2 minutes is greater than the current time. At this time, you need to use the date_add and now functions at the same time. as follows:
2. String functions
Aggregate functions can only appear in the select statement, and the string functions here can appear in both the select statement and the where filter statement.
2.1. Commonly used string functions
Commonly used string functions are as follows:
2.2. charset function
The following employee table exists, and it is required to obtain the character set used by the ename column in the employee table. as follows:
When querying the information in the employee table, use the charset function to obtain the character set used by the ename column. as follows:
2.3.concat function
The following score sheet is currently available, and the information in the score sheet is required to be displayed in the format of "XXX's Chinese is XX points, mathematics is XX points, and English is XX points". as follows:
When querying the information in the grade table, use the concat function to perform string concatenation as required. as follows:
2.4.instr function
The instr function is used to get the position of the first occurrence of a string in another string, and returns 0 if it does not appear. as follows:
2.5. ucase function
The ucase function is used to obtain the string converted to uppercase. as follows:
2.6. lcase function
The lcase function is used to obtain the string converted to lowercase. as follows:
2.7. left function
The left function is used to intercept the specified number of characters backward from the left side of the string. as follows:
2.8. length function
The length function is used to get the number of bytes occupied by the string. as follows:
Note: For multi-byte characters, the number of bytes occupied by a character in different encodings is different. For example, a character in utf8 occupies 3 bytes, while a character in gbk occupies 2 bytes.
2.9.replace function
The replace function is used to replace a specified substring in a string with another string, for example, replace "S" in all names in the employee table with "Shanghai". as follows:
2.10. strcmp function
The strcmp function is used to compare the size of two strings character by character according to the ASCII code. The two strings are equal in size and return 0, the former returns 1, and the latter returns -1. as follows:
It should be noted that the strcmp function is not case-sensitive when comparing. as follows:
2.11. substring function
The substring function is used to intercept the specified number of characters backwards from the specified position of the string. as follows:
When using the substring function, if the number of characters to be intercepted is not specified, it will be intercepted from the specified position to the end by default. as follows:
For example, intercept the second to third characters of the ename field in the employee table. as follows:
2.12.ltrim, rtrim and trim functions
The trim function is used to remove leading and trailing spaces from a string. as follows:
The ltrim and rtrim functions are used to remove the leading and trailing spaces of a string, respectively. as follows:
2.13. Comprehensive case
It is required to display the names of all employees in the employee table in lowercase, the idea is as follows:
• Use the substring function to intercept the first character of the employee's name, and then use lcase to convert it to lowercase.
• Use the substring function to intercept the second and subsequent characters of the employee's name.
• Use the concat function to concatenate the first letter intercepted and converted to lowercase with the string intercepted the second time.
Finally, the string obtained after the concat function is concatenated is the employee name with the first letter lowercase. as follows:
3. Mathematical functions
Aggregate functions can only appear in the select statement, and the mathematical functions here can appear in both the select statement and the where filter statement.
3.1. Commonly used mathematical functions
Commonly used mathematical functions are as follows:
3.2.abs function
The abs function is used to obtain the absolute value of a number. as follows:
3.3.bin function
The bin function is used to convert a decimal number to binary. as follows:
3.4.hex function
The hex function is used to convert a decimal number to hexadecimal. as follows:
3.5.conv function
The conv function is used to convert a number from one base to another base. as follows:
3.6.ceiling function
The ceiling function is used to round up a number. as follows:
It should be noted that the essence of rounding up is to round towards positive infinity, so after negative numbers are rounded up, the first integer greater than or equal to this number is obtained. as follows:
3.7. floor function
The floor function is used to round down a number. as follows:
It should be noted that the essence of rounding down is to round towards negative infinity, so after negative numbers are rounded down, the first integer less than or equal to this number is obtained. as follows:
3.8. format function
The format function is used to format the value, rounding to retain the specified number of decimal places. as follows:
Note: The data after the execution of the format function will be output and displayed (it can be understood that the data after the execution of the format function becomes a string), and the numerical comparison of the data after the execution of the format function cannot be performed, as shown in the figure below.
3.9. rand function
The rand function is used to generate random floating point numbers from 0.0 to 1.0. as follows:
If you want to generate a random number from 0 to 100, you can multiply the generated random floating-point number by 100, and then round it in some way. as follows:
3.10. mod function
The mod function is used to perform remainder operations on numeric values. as follows:
4. Other functions
4.1. user function
The user function is used to get the current username and hostname of the MySQL connection. as follows:
4.2.md5 function
The md5 function is used to perform an md5 digest on a string, and obtain a 32-bit string after the digest. as follows:
expand:
• Generally, the company's internal database does not store the user's plaintext password, but forms a digest of the user password and stores the corresponding digest. When the user logs in to the account, the password entered by the user is digested and compared with the digest stored in the database. Compare, and allow login if the comparison is successful.
• There are two main advantages of doing this. The first advantage is that the company's internal database does not store the user's plaintext information. Even if the user information is leaked, it will not have much impact. Long, this is conducive to the design of the database table structure.
4.3. database function
The database function is used to display the database currently in use. as follows:
4.4.password function
The password function is used to encrypt user data. as follows:
Note: The encryption strength of the password function is higher than that of the md5 function, and mysql uses the password function internally.
4.5. ifnull function
The ifnull function accepts two parameters and returns the value of the first parameter if the first parameter is not null, otherwise returns the value of the second parameter. as follows:
