SELECT COUNT(*) INTO var WHERE CONDITION;

IF var > 0 THEN
SELECT NEEDED_FIELD INTO otherVar WHERE CONDITION;

In PLSQL method with count(*) is unsafe in above code. If another session deletes the row that met the condition after the line with the count(*), and before the line with the select ... into, the code will throw an exception that will not get handled.

Use the below insted,

SELECT NEEDED_FIELD INTO var WHERE CONDITION;
EXCEPTION
WHEN NO_DATA_FOUND

References

Stack Overflow

The SET DEFINE command changes the prefix character used to mark substitution variables. You can use SET DEFINE to turn variable substitution off.

SET DEF[INE] {OFF | ON | prefix_char}

Define is a SQL*Plus client variable. It is NOT a database level setting.

When you start SQL*Plus, variable substitution will be on by default, and the default prefix character is an ampersand. If you are running a script that uses ampersands in text strings, you may want to change the prefix character to something else. If your script doesn’t use substitution variables, you may find it easiest to turn the feature off.

References

Oreilly

The %TYPE attribute lets use the datatype of a field, record, nested table, database column, or variable in your own declarations, rather than hardcoding the type names.

You can use the %TYPE attribute as a datatype specifier when declaring constants, variables, fields, and parameters. If the types that you reference change, your declarations are automatically updated.

This technique saves you from making code changes when, for example, the length of a VARCHAR2 column is increased. Note that column constraints, such as the NOT NULL and check constraint, or default values are not inherited by items declared using %TYPE.