1. Function
1. sysdate : The current date includes time
2. length (str) function: returns the length of the string, str represents a string
3. concat (str1, str2): str1, str2 are both strings, concatenate the strings str1 and str2 together
||: It is a string concatenation character
4. chr(ASCCII): It converts ASCCII code into characters
5. substr (str,index[,len]): string interception function, str is a string, the starting position of index interception, and the length of len interception
6. trim ([s from ]str): str represents a string, s represents a character, which means removing the spaces at both ends of the string. If s is added, it means removing the characters s at both ends of the string.
ltrim( str[,'s'] ): Remove spaces or characters s on the left side of the string
rtrim( str[,'s'] ): Remove spaces or characters s on the right side of the string
7. replace (str, s, d): Replace the function str string, the content to be replaced by s, and replace d with the content, which means replacing the s in str in the character with d.
8.lpad/rpad(str,len[,s]): str represents a string, len represents the length of the expanded string, s: represents the content to be expanded, optional parameters
9.initcap(str): str represents a string, capitalize the first letter of the English word in the string
10.lower/upper(str): Convert all strings to lowercase/uppercase
11.instr(str,s[,n1,n2]): Find the position of a certain character in the string, str string, s: the character to be queried, n1 means starting the search from the position, n2 means the number of this character occurrences
12. floor (n): rounding function, only the integer part is taken, no rounding is performed
13. mod (m,n): a function that finds the remainder of m divided by n
14. round (n,s): rounding function, n represents the number, s represents the precision, the default is 0
15.power(m,n): Find the nth power of m
16. to_number (str[,format]): Convert string to number, str string, format format string
17. to_date (str,format): Convert a string into a date in a fixed format, str is a date, format: date format
yyyy: represents a 4-digit year
MM: represents two months
dd: represents the day of two digits
hh24: indicates the hour in 24-hour format
mi: represents minutes
ss: represents seconds
day: represents the day of the week
18. to_char (date,format): Convert date into string, date is a date, format: date format
19. add_months (date,n): Add and subtract n months to the date
20.months_between(date1,date2): Find the number of months between two dates
21.last_day(date): Get the last day of the current month
22.trunc(): truncation function
23. nvl (variable, default value): If the value of the variable is empty, the function returns the default value. If the value of the variable is not empty, the function returns the value of the variable.
2. Affairs
A transaction (TRANSACTION) is a series of operations performed as a single logical unit of work. These operations are submitted to the system as a whole, and either all or none of them are executed .
Transactions must have the following four attributes, referred to as ACID attributes:
Atomicity: A transaction is a complete operation. Each step of the transaction is indivisible (atomic); either all are executed or none are executed.
Consistency: When the transaction is completed, the data must be in a consistent state
Isolation: All concurrent transactions that modify data are isolated from each other, which indicates that transactions must be independent and should not depend on or affect other transactions in any way
Durability: After a transaction is completed, its modifications to the database are retained permanently, and the transaction log can maintain the permanence of the transaction.
table backup
create table table name as select statement;