SUBSTR returns a substring, specified by y and z, of x.
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x should have CHARACTER type, and if not, it is converted thereto.
y should have FIXED type, and if not, it is converted thereto.
z should have FIXED type, and if not, it is converted thereto.
If z is zero, a null string is returned. If z is omitted, the substring returned is position y in x to the end of x.
z must be nonnegative, and the values of y and z must be such that the substring lies entirely within the current length of x.
If y = LENGTH(x)+1 and z = 0, then the null string is returned.