MySQL - common functions

Character manipulation

Character interception

left(field,length)

left(col_name, length) function : From left to right .col_name Is the column name （ Mandatory ）,length Is the length taken from left to right （ Mandatory As a positive integer , If it is negative, nothing is returned , Subscript from 1 Start , No 0）

right(col_name, length)

right(col_name, length) function : Cut from right to left .col_name Is the column name （ Mandatory ）,length Is the length taken from right to left （ must As a positive integer , If it is negative, nothing is returned Subscript from 1 Start , No 0）

substring(col_name, start, length)

substring(col_name, start, length) function : col_name Name （ Mandatory ）,start From the first few （ Mandatory Integers from 1 Start ）,length Truncated length （ Optional , As a positive integer ）

substring_index(col, a, num)

substring_index(col, a, num) function ： From num individual a To segment col, When num When it is a positive integer , Split from left to right and take from left to right ;num When is a negative integer , Split from right to left and take from right to left .

summary

1、SUBSTRING_INDEX(subStr, matchStr, index）

Start to intercept the string according to the matching character and the position of occurrence

• subStr: Fields to be intercepted
• matchStr: Matched character
• index: From the first few matching characters , When it is a positive number, search from the left and intercept to the left , When it is negative, search from the right and intercept to the right

2、LEFT(subStr,index)

Cut from the left to the right , As of index position （index from 1 Start ）

• subStr: Fields to be intercepted
• index: Where to stop intercepting （ Include characters in this position ）

3、RIGHT(subStr,index)
Cut from the right to the left , Up to... From the right index position （index from 1 Start ）

• subStr: Fields to be intercepted
• index: Where to stop intercepting （ Include characters in this position ）

4、SUBSTRING(subStr, index）
When index When it's negative , Cut from the right to the left , Up to... From the right index position
When index Positive number , Cut from the left to the right , Up to... From the left index position

• subStr: Fields to be intercepted
• index: Where to stop intercepting （ Include characters in this position ）