github.com/dolthub/go-mysql-server@v0.18.0/sql/expression/function/nullif.go (about)

     1  // Copyright 2020-2021 Dolthub, Inc.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package function
    16  
    17  import (
    18  	"fmt"
    19  
    20  	"github.com/dolthub/go-mysql-server/sql"
    21  	"github.com/dolthub/go-mysql-server/sql/expression"
    22  	"github.com/dolthub/go-mysql-server/sql/types"
    23  )
    24  
    25  // NullIf function compares two expressions and returns NULL if they are equal. Otherwise, the first expression is returned.
    26  type NullIf struct {
    27  	expression.BinaryExpressionStub
    28  }
    29  
    30  var _ sql.FunctionExpression = (*NullIf)(nil)
    31  var _ sql.CollationCoercible = (*NullIf)(nil)
    32  
    33  // NewNullIf returns a new NULLIF UDF
    34  func NewNullIf(ex1, ex2 sql.Expression) sql.Expression {
    35  	return &NullIf{
    36  		expression.BinaryExpressionStub{
    37  			LeftChild:  ex1,
    38  			RightChild: ex2,
    39  		},
    40  	}
    41  }
    42  
    43  // FunctionName implements sql.FunctionExpression
    44  func (f *NullIf) FunctionName() string {
    45  	return "nullif"
    46  }
    47  
    48  // Description implements sql.FunctionExpression
    49  func (f *NullIf) Description() string {
    50  	return "returns NULL if expr1 = expr2 is true, otherwise returns expr1."
    51  }
    52  
    53  // Eval implements the Expression interface.
    54  func (f *NullIf) Eval(ctx *sql.Context, row sql.Row) (interface{}, error) {
    55  	if types.IsNull(f.LeftChild) && types.IsNull(f.RightChild) {
    56  		return nil, nil
    57  	}
    58  
    59  	val, err := expression.NewEquals(f.LeftChild, f.RightChild).Eval(ctx, row)
    60  	if err != nil {
    61  		return nil, err
    62  	}
    63  	if b, ok := val.(bool); ok && b {
    64  		return nil, nil
    65  	}
    66  
    67  	return f.LeftChild.Eval(ctx, row)
    68  }
    69  
    70  // Type implements the Expression interface.
    71  func (f *NullIf) Type() sql.Type {
    72  	if types.IsNull(f.LeftChild) {
    73  		return types.Null
    74  	}
    75  
    76  	return f.LeftChild.Type()
    77  }
    78  
    79  // CollationCoercibility implements the interface sql.CollationCoercible.
    80  func (f *NullIf) CollationCoercibility(ctx *sql.Context) (collation sql.CollationID, coercibility byte) {
    81  	if types.IsNull(f.LeftChild) {
    82  		return sql.Collation_binary, 6
    83  	}
    84  	return sql.GetCoercibility(ctx, f.LeftChild)
    85  }
    86  
    87  // IsNullable implements the Expression interface.
    88  func (f *NullIf) IsNullable() bool {
    89  	return true
    90  }
    91  
    92  func (f *NullIf) String() string {
    93  	return fmt.Sprintf("%s(%s,%s)", f.FunctionName(), f.LeftChild, f.RightChild)
    94  }
    95  
    96  // WithChildren implements the Expression interface.
    97  func (f *NullIf) WithChildren(children ...sql.Expression) (sql.Expression, error) {
    98  	if len(children) != 2 {
    99  		return nil, sql.ErrInvalidChildrenNumber.New(f, len(children), 2)
   100  	}
   101  	return NewNullIf(children[0], children[1]), nil
   102  }