Summary of common basic problems in sql (start from small things, don't make such mistakes)

1. Regarding the use of the intercepted string substr, note: this is different from the use of substring in java, as follows, here you need to pay attention to the meaning of the third parameter, which is the number of bits to be intercepted, not the position;

substr (string to be intercepted, start position to intercept, number of bits to intercept);

For example: substr('1234',1,3)='123'; substr('123456789',3,4)='3456'; as shown in the figure:

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