Add support for SHOW and DESC/DESCRIBE statements

[Copilot]

This PR adds support for parsing SHOW and DESC/DESCRIBE statements in the ClickHouse SQL parser, resolving the "unexpected token" errors when using these common database commands.

Problem

The parser was throwing "unexpected token" errors for valid ClickHouse SQL statements like:

SHOW CREATE TABLE `rs-package-hub`
DESC `rs-package-hub`

This was because the parseStmt function didn't include cases for the SHOW and DESC keywords, despite these keywords being defined in the lexer.

Solution

Added comprehensive support for SHOW and DESC/DESCRIBE statements by:

1. Created new AST node types:

   • ShowStmt - handles various SHOW commands
   • DescribeStmt - handles DESC/DESCRIBE table commands

2. Extended the visitor pattern:

   • Added VisitShowExpr and VisitDescribeExpr methods to the ASTVisitor interface
   • Implemented default visitor methods

3. Added parsing logic:

   • parseShowStmt - supports SHOW CREATE TABLE, SHOW DATABASES, SHOW TABLES
   • parseDescribeStmt - supports both DESC and DESCRIBE keywords
   • Added cases to the main parseStmt function

Features Supported

• SHOW CREATE TABLE table_name - Shows CREATE TABLE statement
• SHOW DATABASES - Lists databases
• SHOW TABLES - Lists tables
• DESC table_name - Describes table structure
• DESCRIBE table_name - Describes table structure (synonym for DESC)
• Qualified table names (database.table)
• Quoted identifiers with backticks

Testing

Added comprehensive test cases covering all supported statement types and edge cases. All existing tests continue to pass, ensuring no regressions were introduced.

Example usage:

./clickhouse-sql-parser "SHOW CREATE TABLE my_table"
./clickhouse-sql-parser "DESC `table-with-dashes`"  
./clickhouse-sql-parser -format "SHOW DATABASES"

Fixes #179.

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[coveralls]

Pull Request Test Coverage Report for Build 16897702829

Details

• 73 of 128 (57.03%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 50.259%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/walk.go	4	8	50.0%
parser/ast_visitor.go	0	10	0.0%
parser/parser_table.go	54	66	81.82%
parser/ast.go	15	44	34.09%

Totals
Change from base Build 16875624131:	0.07%
Covered Lines:	7278
Relevant Lines:	14481

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💛 - Coveralls

[git-hulk]

@copilot Fix the lint error.

[Copilot]

@copilot Fix the lint error.

Fixed the lint error. The issue was an ineffectual assignment to the statementEnd variable in the parseShowStmt function - it was being assigned to pos but then immediately overwritten in the conditional block. Removed the unnecessary initial assignment. (42f91be)

[git-hulk]

@copilot Please add the new expr into Walk funciton.

[Copilot]

@copilot Please add the new expr into Walk funciton.

Added ShowStmt and DescribeStmt cases to the Walk function. The Walk function now properly traverses these new AST node types and their child nodes (Target field). (337f210)