Add oracle select union statement parse test

[zihaoAK47]

Fixes #ISSUSE_ID.

Changes proposed in this pull request:

• Add complex select union query statement parsing test
• Add multiple union statement parsing test
• Add multiple union priority test
• Priority Description Reference link
  

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[strongduanmu]

Hi @zihaoAK47, can you show antlr preview for more than one union statement? It seems that the priority processing of multiple Union statements is wrong.

[zihaoAK47]

Hi @zihaoAK47, can you show antlr preview for more than one union statement? It seems that the priority processing of multiple Union statements is wrong.

Hi mentor, I have submitted a new parsing code.
The following is a preview image of Antlr parsing

[strongduanmu]

Hi @zihaoAK47, can you show antlr preview for more than one union statement? It seems that the priority processing of multiple Union statements is wrong.

Hi mentor, I have submitted a new parsing code. The following is a preview image of Antlr parsing

@zihaoAK47 Now the result of sql parser is correct, thank you for correct it.