Essential amino acids also have codons.Still they are needed by humans through diet?Will those codons do synthesize amino acids for some other purposes in humans or not?If so, what is the purpose?e.g?
You are mixing things. First: Codons have nothing to do with the synthesis of amino acids.
A codon is the sequence of nucleotides on the DNA (or RNA) which code for a certain amino acid. For example UGU and UGC code for the amino acid Cysteine and given these codons it will appear in the protein being synthesized. The codons are basically a way to code the sequence of a protein with the smallest amount of information (simply by the sequence of the four nucleotides in DNA/RNA). For further information have a look at the Wikipedia page on Codons.
This look like this (image from the Wikipedia):
Answer to the comment
Partial Amino acid sequence of alpha amylase in Homo sapiens:
Below s the amino acid single letter representation table that can be referred to understand the amino acid sequence
Make sure you follow the reading frame scheme to read the nucleotide sequence. Each time frame should have a triplet codon without including the previous nucleotide. I've represented it for the first few codons. You can observe essential amino acids in this protein. All the amino acids in this partial sequence are essential except glycine. Hope this helps :)