Explain the Use of UNION, SELECT, WAIT and --

1. UNION
   The UNION operator is used to combine the result-set of two or more SELECT statements.
   • Every SELECT statement within UNION must have the same number of columns
   • The columns must also have similar data types
   • The columns in every SELECT statement must also be in the same order
     
2. SELECT
   The SELECT statement is used to select data from a database. The data returned is stored in a result table, called the result-set
   
3. WAIT
   Hmm, about WAIT, we’re going to use SLEEP because WAIT command is not available in MySQL. SLEEP is a command to put a delay on an output before it gets returned.
   
4. --
-- is one of three comment styles that available in MySQL
   

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