Fix 5 parser tests with LIMIT BY, MySQL @@ vars, and LIKE alias support

ast/ast.go

@@ -70,8 +70,10 @@ type SelectQuery struct {
 	Qualify     Expression            `json:"qualify,omitempty"`
 	Window      []*WindowDefinition   `json:"window,omitempty"`
 	OrderBy     []*OrderByElement     `json:"order_by,omitempty"`
-	Limit       Expression            `json:"limit,omitempty"`
-	Offset      Expression            `json:"offset,omitempty"`
+	Limit            Expression            `json:"limit,omitempty"`
+	LimitBy          []Expression          `json:"limit_by,omitempty"`
+	LimitByHasLimit  bool                  `json:"limit_by_has_limit,omitempty"` // true if LIMIT BY was followed by another LIMIT
+	Offset           Expression            `json:"offset,omitempty"`
 	Settings    []*SettingExpr        `json:"settings,omitempty"`
 	IntoOutfile *IntoOutfileClause    `json:"into_outfile,omitempty"`
 	Format      *Identifier           `json:"format,omitempty"`
@@ -1067,11 +1069,12 @@ func (i *IsNullExpr) expressionNode()     {}
 
 // LikeExpr represents a LIKE or ILIKE expression.
 type LikeExpr struct {
-	Position    token.Position `json:"-"`
-	Expr        Expression     `json:"expr"`
-	Not         bool           `json:"not,omitempty"`
-	CaseInsensitive bool       `json:"case_insensitive,omitempty"` // true for ILIKE
-	Pattern     Expression     `json:"pattern"`
+	Position        token.Position `json:"-"`
+	Expr            Expression     `json:"expr"`
+	Not             bool           `json:"not,omitempty"`
+	CaseInsensitive bool           `json:"case_insensitive,omitempty"` // true for ILIKE
+	Pattern         Expression     `json:"pattern"`
+	Alias           string         `json:"alias,omitempty"`
 }
 
 func (l *LikeExpr) Pos() token.Position { return l.Position }

internal/explain/format.go

@@ -21,12 +21,16 @@ func FormatFloat(val float64) string {
 	if math.IsNaN(val) {
 		return "nan"
 	}
-	// Use scientific notation for extremely small numbers (< 1e-10)
-	// This matches ClickHouse's behavior where numbers like 0.000001 stay decimal
-	// but extremely small numbers like 1e-38 use scientific notation
+	// Use scientific notation for very small numbers (< 1e-6)
+	// This matches ClickHouse's behavior where numbers like 0.0000001 (-1e-7)
+	// are displayed in scientific notation
 	absVal := math.Abs(val)
-	if absVal > 0 && absVal < 1e-10 {
-		return strconv.FormatFloat(val, 'e', -1, 64)
+	if absVal > 0 && absVal < 1e-6 {
+		s := strconv.FormatFloat(val, 'e', -1, 64)
+		// Remove leading zeros from exponent (e-07 -> e-7)
+		s = strings.Replace(s, "e-0", "e-", 1)
+		s = strings.Replace(s, "e+0", "e+", 1)
+		return s
 	}
 	// Use decimal notation for normal-sized numbers
 	return strconv.FormatFloat(val, 'f', -1, 64)

internal/explain/functions.go

@@ -655,7 +655,11 @@ func explainLikeExpr(sb *strings.Builder, n *ast.LikeExpr, indent string, depth
 	if n.Not {
 		fnName = "not" + strings.Title(fnName)
 	}
-	fmt.Fprintf(sb, "%sFunction %s (children %d)\n", indent, fnName, 1)
+	if n.Alias != "" {
+		fmt.Fprintf(sb, "%sFunction %s (alias %s) (children %d)\n", indent, fnName, n.Alias, 1)
+	} else {
+		fmt.Fprintf(sb, "%sFunction %s (children %d)\n", indent, fnName, 1)
+	}
 	fmt.Fprintf(sb, "%s ExpressionList (children %d)\n", indent, 2)
 	Node(sb, n.Expr, depth+2)
 	Node(sb, n.Pattern, depth+2)

internal/explain/select.go

@@ -113,6 +113,13 @@ func explainSelectQuery(sb *strings.Builder, n *ast.SelectQuery, indent string,
 	if n.Limit != nil {
 		Node(sb, n.Limit, depth+1)
 	}
+	// LIMIT BY - only output when there's no ORDER BY and no second LIMIT (matches ClickHouse behavior)
+	if len(n.LimitBy) > 0 && len(n.OrderBy) == 0 && !n.LimitByHasLimit {
+		fmt.Fprintf(sb, "%s ExpressionList (children %d)\n", indent, len(n.LimitBy))
+		for _, expr := range n.LimitBy {
+			Node(sb, expr, depth+2)
+		}
+	}
 	// SETTINGS - output here if there's no FORMAT, otherwise it's at SelectWithUnionQuery level
 	if len(n.Settings) > 0 && n.Format == nil {
 		fmt.Fprintf(sb, "%s Set\n", indent)
@@ -195,6 +202,16 @@ func isComplexExpr(expr ast.Expression) bool {
 	}
 }
 
+// hasOnlyLiterals checks if all expressions in a slice are literals
+func hasOnlyLiterals(exprs []ast.Expression) bool {
+	for _, expr := range exprs {
+		if _, ok := expr.(*ast.Literal); !ok {
+			return false
+		}
+	}
+	return true
+}
+
 func countSelectUnionChildren(n *ast.SelectWithUnionQuery) int {
 	count := 1 // ExpressionList of selects
 	// Check if any SelectQuery has IntoOutfile set
@@ -259,6 +276,9 @@ func countSelectQueryChildren(n *ast.SelectQuery) int {
 	if n.Limit != nil {
 		count++
 	}
+	if len(n.LimitBy) > 0 && len(n.OrderBy) == 0 && !n.LimitByHasLimit {
+		count++
+	}
 	if n.Offset != nil {
 		count++
 	}

parser/expression.go

@@ -419,6 +419,33 @@ func (p *Parser) parseIdentifierOrFunction() ast.Expression {
 	name := p.current.Value
 	p.nextToken()
 
+	// Check for MySQL-style @@variable syntax (system variables)
+	// Convert to globalVariable('varname') function call with alias @@varname
+	if strings.HasPrefix(name, "@@") {
+		varName := name[2:] // Strip @@
+		// Handle @@session.var or @@global.var
+		if p.currentIs(token.DOT) {
+			p.nextToken()
+			if p.currentIs(token.IDENT) || p.current.Token.IsKeyword() {
+				varName = varName + "." + p.current.Value
+				name = name + "." + p.current.Value
+				p.nextToken()
+			}
+		}
+		return &ast.FunctionCall{
+			Position: pos,
+			Name:     "globalVariable",
+			Alias:    name,
+			Arguments: []ast.Expression{
+				&ast.Literal{
+					Position: pos,
+					Type:     "String",
+					Value:    varName,
+				},
+			},
+		}
+	}
+
 	// Check for function call
 	if p.currentIs(token.LPAREN) {
 		return p.parseFunctionCall(name, pos)
@@ -1591,6 +1618,9 @@ func (p *Parser) parseAlias(left ast.Expression) ast.Expression {
 	case *ast.ExtractExpr:
 		e.Alias = alias
 		return e
+	case *ast.LikeExpr:
+		e.Alias = alias
+		return e
 	default:
 		return &ast.AliasedExpr{
 			Position: left.Pos(),

parser/parser.go

@@ -444,9 +444,10 @@ func (p *Parser) parseSelect() *ast.SelectQuery {
 		// LIMIT BY clause (ClickHouse specific: LIMIT n BY expr1, expr2, ...)
 		if p.currentIs(token.BY) {
 			p.nextToken()
-			// Parse LIMIT BY expressions - skip them for now
+			// Parse LIMIT BY expressions
 			for !p.isEndOfExpression() {
-				p.parseExpression(LOWEST)
+				expr := p.parseExpression(LOWEST)
+				sel.LimitBy = append(sel.LimitBy, expr)
 				if p.currentIs(token.COMMA) {
 					p.nextToken()
 				} else {
@@ -457,6 +458,7 @@ func (p *Parser) parseSelect() *ast.SelectQuery {
 			if p.currentIs(token.LIMIT) {
 				p.nextToken()
 				sel.Limit = p.parseExpression(LOWEST)
+				sel.LimitByHasLimit = true
 			}
 		}
 

parser/testdata/00176_distinct_limit_by_limit_bug_43377/ast.json

@@ -94,7 +94,15 @@
                       "limit": {
                         "type": "Integer",
                         "value": 10
-                      }
+                      },
+                      "limit_by": [
+                        {
+                          "parts": [
+                            "Title"
+                          ]
+                        }
+                      ],
+                      "limit_by_has_limit": true
                     }
                   ]
                 }

parser/testdata/00583_limit_by_expressions/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/00590_limit_by_column_removal/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/01337_mysql_global_variables/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/02045_like_function/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/02281_limit_by_distributed/ast.json

@@ -98,7 +98,14 @@
                       "limit": {
                         "type": "Integer",
                         "value": 1
-                      }
+                      },
+                      "limit_by": [
+                        {
+                          "parts": [
+                            "k"
+                          ]
+                        }
+                      ]
                     }
                   ]
                 }

parser/testdata/03213_rand_dos/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/03366_with_fill_dag/ast.json

@@ -48,7 +48,14 @@
       "limit": {
         "type": "Integer",
         "value": 1
-      }
+      },
+      "limit_by": [
+        {
+          "parts": [
+            "number"
+          ]
+        }
+      ]
     }
   ]
 }